I wanted to remove the split() entirely, but my knowledge of Perl isn't strong enought yet to come up with that solution. Here's a slight improvement that I came up with.

#!/usr/bin/perl

   use strict;

   my ($full, $part) = @ARGV;

   my ($left) = leftover($full, $part);

   if (defined($left)) { print "$full - $part = $left\n"; }
   else                { print "$full - $part = undef\n"; }

#-----------------------------------------------------------
sub leftover {

   my ($full, $part) = @_;
   my ($ch);
   my (@parts) = split('', $part);
   my ($regx) = join('+.*', @parts);

   if ($full =~ m{$regx}) {
       foreach $ch (@parts)
       {
           $full =~ s/[$ch]{1}//;
       }
   }
   else { $full = undef; };

   return $full;
}
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Here's pseudo code for what my original idea was (maybe someone else can make it work):

sub leftover{
  my ($full, $part) = @_;
  while ($full && $part)
  {
    #RexExp to substitute nothing for the first match
    #then remove the matched character from $part
  }
}
[download]