Hidden features of Perl?

Answer 1

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/

Answer 2

The continue clause on loops. It will be executed at the bottom of every loop, even those which are next'ed.

while( <> ){
  print "top of loop\n";
  chomp;

  next if /next/i;
  last if /last/i;

  print "bottom of loop\n";
}continue{
  print "continue\n";
}

Answer 3

Taint checking. With taint checking enabled, perl will die (or warn, with -t) if you try to pass tainted data (roughly speaking, data from outside the program) to an unsafe function (opening a file, running an external command, etc.). It is very helpful when writing setuid scripts or CGIs or anything where the script has greater privileges than the person feeding it data.

Magic goto. goto &sub does an optimized tail call.

The debugger.

use strict and use warnings. These can save you from a bunch of typos.

Answer 4

Based on the way the "-n" and "-p" switches are implemented in Perl 5, you can write a seemingly incorrect program including }{:

ls |perl -lne 'print $_; }{ print "$. Files"'

which is converted internally to this code:

LINE: while (defined($_ = <ARGV>)) {
    print $_; }{ print "$. Files";
}

Answer 5

map - not only because it makes one's code more expressive, but because it gave me an impulse to read a little bit more about this "functional programming".

Answer 6

The operators ++ and unary - don't only work on numbers, but also on strings.

my $_ = "a"
print -$_

prints -a

print ++$_

prints b

$_ = 'z'
print ++$_

prints aa