use strict;
my $str = '<section className={class-a}>
Un-touched stuff here
</section>
<hr />
<section className={class-b}>
Un-touched stuff here
</section>
<hr />';
my $regex = qr/<section\s+className\=\{([^>]*)\}>([\s\S]*?)<\/section>\s*/mp;
my $subst = '<!-- $1 -->\\n$2';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html