This was fun. :)

82 with extra credit, does it in a clever way

#!/usr/bin/perl

use strict;
my $t = shift || "gttb";
my $s = shift || "gattacababy";

my $return = &PopPopChopChop($s,$t);
print "Original: $s\n";
$return = "UNDEF" unless defined $return;
print "Returned: $return\n";
exit;

sub PopPopChopChop {
$a=reverse pop;$_=pop;$b=chop$a;
s!(.)!$b and$1=~/$b/and$b=chop$a;$1!eg;$b?undef:$_
}
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