Yet Another Attempt. Hefty though, weighs in at 110 characters, is strict and warnings compliant and solves extra credit though...

sub x{
   my($s,$t,$p)=(@_,0);
   (join'',map{$s=~/$_/g?($p<pos$s?($p=pos$s and$s=~s/$_//and$_):''):'
+'}split//,$t)eq$t?$s:undef
}
[download]

i'd be delighted to see someone trim it down...

nuf evah,
jynx