use strict;
my $str = '<ins class="thisisfirstscript"
style="display:inline-block;width:336px;height:280px"
data-client="ABCD"
data-slot="4397140188"></ins>
<ins class="thisissecondscript"
style="display:inline-block;width:336px;height:280px"
data-client="CDEF"
data-slot="4496122889"></ins>
<ins class="thisisfirstscript"
style="display:inline-block;width:336px;height:280px"
data-client="ABCD"
data-slot="4397140188"></ins>';
my $regex = qr/(?:<ins class="thisisfirstscript")([\s\S]*?)(?:data-slot="4397140188"><\/ins>)/mp;
my $subst = '$1index.php';
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