use strict;
my $str = '<div class="parent-first-class">
<div class="child-same-class">
<div class="parent-second-class">
<div class="child-same-class">
<div class="parent-third-class">
<div class="child-same-class">';
my $regex = qr#(<div class="([^"]+)">.+<div class="child-same-class">)#isp;
my $subst = '$1\\n\\t<xf:if is="\\$someVariable == \'$2\'">...</xf:if>';
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