I'm not sure whether you can eval a function in a regex. If I understand your question correctly, you can just do

sub search_equivalent{
   #do stuff here and return your equivalent pattern
   return $equivalent;
}

my $equiv = search_equivalent();
if ($thisstring =~ m/$equiv/){
   ....
}
[download]

There is no real need to put a function call in the regex.

Jorg

"Do or do not, there is no try" -- Yoda