my $reg = '99';
my $replace = 'XX';

my $line = "0165451234599123459935589";
print $line, "\n";

$line =~ s/$reg/$replace/;
print $line, "\n";

$line = "0165451234599123459935589";
$line =~ s/(12345)($reg)(12345)/$1$replace$3/;

print $line, "\n";

##</code><code>##

0165451234599123459935589
01654512345XX123459935589
01654512345XX123459935589