I have removed any "filler" statements (i.e. $~=~112;) and comments. I have also formatted the script for readability and added in explanatory comments.

#!/usr/bin/perl -w
use strict;

# clear out $~
$~=~s/.*//;

sub ___{
  # return this string
  'chr(';
}

sub __{
  # return this string
  'print(';
}

sub _{
  # foreach $_ (0 .. $#_) {
  for (0..$#_) {
    # append 'print(chr(' and the $_th element of @_ to $~
    $~ .= __.___.$_[$_];
    # append '))' to $@
    $@ .= '))';
    # if $_ matches the value of $#_ then you are done
    last if /$#_/;
    # append a '+' to $~
    $~ .= '+';
  }
  # the double quoted string "$~$@;" results in a value of
  # print(chr(9+print(chr(71+print(chr(79+print(chr(64+print(chr(74)))
+)))))));
  # eval the string to produce the output
  eval "$~$@;";
}

# call the _ subroutine with the values given
# they are in octal so change them to decimal
# and they are (9,71,79,64,74)
_(0011,0107,0117,0100,0112);
[download]