another way probably

if (/^[[:allnum:]]{1,13}$/){
              =~ s/^([[:allnum:]]{1,13})$/$1.'X'/
}
[download]

to be used inside a loop

I need to include a "X" after the initial letters.

You could print the X in any position you want. i.e, after the first letter

if (/^[[:allnum:]]{1,13}$/){
           =~ s/^([[:allnum:]]{1})([[:allnum:]]{1,12})$/$s1 .'X'. $2/
}
[download]