use stric;
use warnings;

my @files = qw( XY201308.TXT TW201308.TXT KG201308.TXT MM201308.TXT );


foreach my $file (@files) {

  # triyng to lower case the to letters and replace the digits with yy

  $file=~s/(\w{2})(\d+)/$1\lc($1),$2\lc(yy)/e;

  print "\n $file \n";
  
}