my $sensitive   = qr/one/;
my $insensitive = qr/one/i;
foreach (@words) {
   my $pattern = some_condition( $_ ) ? $sensitive : $insensitive;
   if (/$pattern/) {
      print "Matched: pattern was $pattern and token was $_\n";
   }
   else {
      print "Unmatched: pattern was $pattern and token was $_\n";
   }
}