index is much faster than m// when matching a constant string. I'm guessing index + substr is similarly faster than s///. Feel free to benchmark the two.

By the way,
$string1 =~ s/($string2)/<b>$1<\/b>/g;
should be replaced with
$string1 =~ s/(\Q$string2\E)/<b>$1<\/b>/g;
in case $string2 contains special characters. Better yet,
$string1 =~ s/((?:\Q$string2\E)+)/<b>$1<\/b>/g;
should be a little bit faster.

I'd also Benchmark the following which has fewer concatenations:

$string1 =~ s#(?=(?:\Q$string2\E)+)#<b>#g;
$string1 =~ s#(?<=(?:\Q$string2\E)+)#</b>#g;
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