Your sample code doesn't make much sense in that you are not applying the s/// to anything.
You say you want to pass the results of the s/// to the sub, without recourse to a temporary var. If your intent is to apply the s/// to $word, then this requires no additional vars.
$word = 'BLAHBLAH';
$word =~ s/BLAH/COW/;
print blah( $word );
It's not clear to me if this is what you want, but if it's not, maybe you could clarify for me?
Examine what is said, not who speaks.
The 7th Rule of perl club is -- pearl clubs are easily damaged. Use a diamond club instead.