$s = '
<p>something something</p>
<h2>blah blah blah</h2>
<p>something something</p>
';

$before = '<h2>([^<]+)</h2>';
$after = '<h1>$1</h1>';

$s =~ s/$before/$after/;

print $s;

##</code><code>##

<p>something something</p>
<h1>$1</h1>
<p>something something</p>

##</code><code>##

$s =~ s/$before/eval{$after}/e;

##</code><code>##

$regex = 's!<h2>([^<]+)</h2>!<h1>$1</h1>!g';

$s =~ $regex;

##</code><code>##

($_='kkvvttuubbooppuuiiffssqqffssmmiibbddllffss')
 =~y~b-v~a-z~s;                             print