You might also try this one for speed, which avoids building the list unnecessarily, and uses arrays instead of hashes to avoid the hashing algorithm for just a single character:

sub leftover {
  my ($full, $part) = @_;
  my @count;
  $count[ord $1]++ while $full =~ /(\w)/g;
  while ($part =~ /(\w)/g) {
    return undef if --$count[ord $1] < 0;
  }
  my $return;
  $count[$_] and $return .= chr($_) x $count[$_] for 0..$#count;
  $return;
}
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-- Randal L. Schwartz, Perl hacker