Hi,
if you were just after the first '99' in your string, just omit the 'g'-Modifier, otherwise use more groupings to do it:

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";
[download]

prints

0165451234599123459935589
01654512345XX123459935589
01654512345XX123459935589
[download]

Regards,
svenXY