Having a hard time understanding exactly what you mean so I'm going to make a big assumption here. I'm thinking that you have a string 'Billiard' and you want to change that to 'Bill':

if ($line =~ /$partialword/i) {
   # I'm assuming $partialword is 'Bill'...
   $line =~ s/Billiard/Bill/;
}
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-- vek --