use strict;

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

my $start = '<h2>';
my $end = '</h2>';

my $first = index($s,$start);
my $last = index($s,$end,$first);

substr($s, $first, $last-$first+length $end) = '<h1>'.substr($s,$first+length $start,$last-$first-length $start).'</h1>';

print $s;