NAME

DESCRIPTION

can help make your program more bullet-proof, but sometimes it's too annoying for quick throw-away programs.

Awk Traps

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The English module, loaded via

use English;

allows you to refer to special variables (like $RS ) as though they were in awk; see the perlvar manpage for details.

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Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.

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Curly brackets are required on ifs and whiles.

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Variables begin with "$" or "@" in Perl.

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Arrays index from 0. Likewise string positions in substr() and index() .

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You have to decide whether your array has numeric or string indices.

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Associative array values do not spring into existence upon mere reference.

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You have to decide whether you want to use string or numeric comparisons.

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Reading an input line does not split it for you. You get to split it yourself to an array. And split() operator has different arguments.

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The current input line is normally in $_ , not $0 . It generally does not have the newline stripped. ( $0 is the name of the program executed.) See the perlvar manpage .

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<digit> does not refer to fields--it refers to substrings matched by the last match pattern.

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The print() statement does not add field and record separators unless you set $, and $. . You can set $OFS and $ORS if you're using the English module.

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You must open your files before you print to them.

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The range operator is "..", not comma. The comma operator works as in C.

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The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.)

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The exponentiation operator is "**", not "^". "^" is the XOR operator, as in C. (You know, one could get the feeling that awk is basically incompatible with C.)

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The concatenation operator is ".", not the null string. (Using the null string would render /pat/ /pat/ unparsable, since the third slash would be interpreted as a division operator--the tokener is in fact slightly context sensitive for operators like "/", "?", and ">". And in fact, "." itself can be the beginning of a number.)

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The next , exit , and continue keywords work differently.

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The following variables work differently:

• Awk Perl
• ARGC $#ARGV or scalar @ARGV
• ARGV[0] $0
• FILENAME $ARGV
• FNR $. - something
• FS (whatever you like)
• NF $#Fld, or some such
• NR $.
• OFMT $#
• OFS $,
• ORS $\
• RLENGTH length($&)
• RS $/
• RSTART length($`)
• SUBSEP $;

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You cannot set $RS to a pattern, only a string.

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When in doubt, run the awk construct through a2p and see what it gives you.

C Traps

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Curly brackets are required on if's and while's.

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You must use elsif rather than else if.

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The break and continue keywords from C become in Perl last and next , respectively. Unlike in C, these do NOT work within a do { } while construct.

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There's no switch statement. (But it's easy to build one on the fly.)

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Variables begin with "$" or "@" in Perl.

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printf() does not implement the "*" format for interpolating field widths, but it's trivial to use interpolation of double-quoted strings to achieve the same effect.

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Comments begin with "#", not "/*".

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You can't take the address of anything, although a similar operator in Perl 5 is the backslash, which creates a reference.

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ARGV must be capitalized.

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System calls such as link() , unlink() , rename() , etc. return nonzero for success, not 0.

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Signal handlers deal with signal names, not numbers. Use kill -l to find their names on your system.

Sed Traps

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Backreferences in substitutions use "$" rather than "\".

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The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front.

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The range operator is ..., rather than comma.

Shell Traps

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The backtick operator does variable interpretation without regard to the presence of single quotes in the command.

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The backtick operator does no translation of the return value, unlike csh.

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Shells (especially csh) do several levels of substitution on each command line. Perl does substitution only in certain constructs such as double quotes, backticks, angle brackets, and search patterns.

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Shells interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for BEGIN blocks, which execute at compile time).

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The arguments are available via @ARGV , not $1, $2, etc.

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The environment is not automatically made available as separate scalar variables.

Perl Traps

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Remember that many operations behave differently in a list context than they do in a scalar one. See the perldata manpage for details.

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Avoid barewords if you can, especially all lower-case ones. You can't tell just by looking at it whether a bareword is a function or a string. By using quotes on strings and parens on function calls, you won't ever get them confused.

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You cannot discern from mere inspection which built-ins are unary operators (like chop() and chdir() ) and which are list operators (like print() and unlink() ). (User-defined subroutines can only be list operators, never unary ones.) See the perlop manpage .

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People have a hard time remembering that some functions default to $_ , or @ARGV , or whatever, but that others which you might expect to do not.

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The<FH> construct is not the name of the filehandle, it is a readline operation on that handle. The data read is only assigned to $_ if the file read is the sole condition in a while loop:

while (<FH>) { } while ($_ = <FH>) { }.. <FH>; # data discarded!

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Remember not to use "=" when you need "=~"; these two constructs are quite different:

$x = /foo/; $x =~ /foo/;

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The do {} construct isn't a real loop that you can use loop control on.

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Use my() for local variables whenever you can get away with it (but see the perlform manpage for where you can't). Using local() actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping.

Perl4 Traps

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@ now always interpolates an array in double-quotish strings. Some programs may now need to use backslash to protect any @ that shouldn't interpolate.

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Barewords that used to look like strings to Perl will now look like subroutine calls if a subroutine by that name is defined before the compiler sees them. For example:

sub SeeYa { die "Hasta la vista, baby!" } $SIG{'QUIT'} = SeeYa;

In Perl 4, that set the signal handler; in Perl 5, it actually calls the function! You may use the -w switch to find such places.

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Symbols starting with _ are no longer forced into package main, except for $_ itself (and @_, etc.).

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s'$lhs'$rhs' now does no interpolation on either side. It used to interpolate $lhs but not $rhs.

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The second and third arguments of splice() are now evaluated in scalar context (as the book says) rather than list context.

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These are now semantic errors because of precedence:

shift @list + 20; $n = keys %map + 20;

Because if that were to work, then this couldn't:

sleep $dormancy + 20;

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open FOO || die is now incorrect. You need parens around the filehandle. While temporarily supported, using such a construct will generate a non-fatal (but non-suppressible) warning.

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The elements of argument lists for formats are now evaluated in list context. This means you can interpolate list values now.

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You can't do a goto into a block that is optimized away. Darn.

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It is no longer syntactically legal to use whitespace as the name of a variable, or as a delimiter for any kind of quote construct. Double darn.

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The caller() function now returns a false value in a scalar context if there is no caller. This lets library files determine if they're being required.

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m//g now attaches its state to the searched string rather than the regular expression.

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reverse is no longer allowed as the name of a sort subroutine.

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taintperl is no longer a separate executable. There is now a -T switch to turn on tainting when it isn't turned on automatically.

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Double-quoted strings may no longer end with an unescaped $ or @.

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The archaic while/if BLOCK BLOCK syntax is no longer supported.

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Negative array subscripts now count from the end of the array.

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The comma operator in a scalar context is now guaranteed to give a scalar context to its arguments.

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The ** operator now binds more tightly than unary minus. It was documented to work this way before, but didn't.

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Setting $# array lower now discards array elements.

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delete() is not guaranteed to return the old value for tie() d arrays, since this capability may be onerous for some modules to implement.

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The construct "this is $$ x" used to interpolate the pid at that point, but now tries to dereference $x. $$ by itself still works fine, however.

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Some error messages will be different.

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Some bugs may have been inadvertently removed.