PERLTRAP

# NAME

perltrap - Perl traps for the unwary

# DESCRIPTION

The biggest trap of all is forgetting to use the -w switch; see the perlrun manpage . Making your entire program runnable under

        use strict;

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

## Awk Traps

Accustomed awk users should take special note of the following:

*
  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.

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

*
  Curly brackets are required on `if`s and `while`s.

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

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

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

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

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

*
  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.

*
  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 .

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

*
  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.

*
  You must open your files before you print to them.

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

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

*
  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.)

*
  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.)

*
  The next , exit , and `continue` keywords work differently.

*

  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 $;

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

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

.

## C Traps

Cerebral C programmers should take note of the following:

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

*
  You must use `elsif` rather than `else if`.

*
  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.

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

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

*
  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.

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

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

*
  `ARGV` must be capitalized.

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

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

.

## Sed Traps

Seasoned sed programmers should take note of the following:

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

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

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

.

## Shell Traps

Sharp shell programmers should take note of the following:

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

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

*
  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.

*
  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).

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

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

.

## Perl Traps

Practicing Perl Programmers should take note of the following:

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

*
  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.

*
  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 .

*
  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.

*
  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!

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

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

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

*
  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

Penitent Perl 4 Programmers should take note of the following incompatible changes that occurred between release 4 and release 5:

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

*
  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.

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

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

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

*
  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;

*
  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.

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

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

*
  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.

*
  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.

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

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

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

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

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

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

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

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

*
  Setting `$# array` lower now discards array elements.

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

*
  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.

*
  Some error messages will be different.

*
  Some bugs may have been inadvertently removed.

.