Hidden features of Perl?

Question

The flip-flop operator is useful for skipping the first iteration when looping through the records (usually lines) returned by a file handle, without using a flag variable:

while(<$fh>)
{
  next if 1..1; # skip first record
  ...
}

Run perldoc perlop and search for "flip-flop" for more information and examples.

There are many non-obvious features in Perl.

For example, did you know that there can be a space after a sigil?

 $ perl -wle 'my $x = 3; print $ x'
 3

Or that you can give subs numeric names if you use symbolic references?

$ perl -lwe '*4 = sub { print "yes" }; 4->()' 
yes

There's also the "bool" quasi operator, that return 1 for true expressions and the empty string for false:

$ perl -wle 'print !!4'
1
$ perl -wle 'print !!"0 but true"'
1
$ perl -wle 'print !!0'
(empty line)

Other interesting stuff: with use overload you can overload string literals and numbers (and for example make them BigInts or whatever).

Many of these things are actually documented somewhere, or follow logically from the documented features, but nonetheless some are not very well known.

Update: Another nice one. Below the q{...} quoting constructs were mentioned, but did you know that you can use letters as delimiters?

$ perl -Mstrict  -wle 'print q bJet another perl hacker.b'
Jet another perl hacker.

Likewise you can write regular expressions:

m xabcx
# same as m/abc/

Add support for compressed files via magic ARGV:

s{ 
    ^            # make sure to get whole filename
    ( 
      [^'] +     # at least one non-quote
      \.         # extension dot
      (?:        # now either suffix
          gz
        | Z 
       )
    )
    \z           # through the end
}{gzcat '$1' |}xs for @ARGV;

(quotes around $_ necessary to handle filenames with shell metacharacters in)

Now the <> feature will decompress any @ARGV files that end with ".gz" or ".Z":

while (<>) {
    print;
}

One of my favourite features in Perl is using the boolean || operator to select between a set of choices.

 $x = $a || $b;

 # $x = $a, if $a is true.
 # $x = $b, otherwise

This means one can write:

 $x = $a || $b || $c || 0;

to take the first true value from $a, $b, and $c, or a default of 0 otherwise.

In Perl 5.10, there's also the // operator, which returns the left hand side if it's defined, and the right hand side otherwise. The following selects the first defined value from $a, $b, $c, or 0 otherwise:

$x = $a // $b // $c // 0;

These can also be used with their short-hand forms, which are very useful for providing defaults:

$x ||= 0;   # If $x was false, it now has a value of 0.

$x //= 0;   # If $x was undefined, it now has a value of zero.

Cheerio,

Paul