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( $word );
print "$word\n";
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It's not clear to me if this is what you want, but if it's not, maybe you could clarify for me?