This is some decoding script for form input. You should not be doing this yourself. See Use CGI or die; and No excuses about not using CGI.pm

Anyway here is an explanation:

# iterate over @pairs array assigning each value to $pair in turn
# this should start foreach my $pair (@pairs) 
# see Use strict warnings and diagnostics

foreach $pair (@pairs){

# this splits each pair into $name and $value at the "="
# the "=" is thrown away

      ($name,$value) = split(/=/,$pair);

# this tr(ansliterates) the "+" chars into spaces
# in HTTP this is how spaces are encoded

      $value =~ tr/+/ /;

# this s(ubstitutes) some special encoded characters into their ASCII values
# In HTTP certain chars have special meaning to the protocol so need
# to be encoded. These are encoded as their hex equivalent
# The encoded form is %DD where the DD are two hex digits and
# the leading % is a flag to let us know the next two chars are 
# hex digits encoding a ASCII literal. Now to the regex itself.
# the first part captures the two hex digits after the % into $1
# the second part converts these to their ASCII equivalents
# the /eg at the end means "e" evaluate the second part and 
# "g" do globally ie all ocurrences

      $value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C",hex($1))/eg;
      

# this last line generates a associative array (a hash) called %VALUE
# the key in this hash is $name and the value is $value

      $VALUE{$name} = $value;

I recommend you check out Ovid's Web Programming with Perl tutorial for more details. This code will choke on names like "Jack&Jill" or "John Smith" because the decoding is only performed on $value, not $name. These will remain encoded as "Jack%26Jill" and "John+Smith".

cheers

tachyon