#!/usr/bin/perl
use strict; 

my $string = "CATINTHEHATWITHABAT";
my $regex = '\wAT';
my @matches = ();

while ($string =~ s/($regex)/lc($1)/e){
    my $match = $1;
    my $length = length($&);
    my $pos = length($`);
    my $start = $pos + 1;
    my $end = $pos + $length;
    my $hitpos = "$start-$end";
    push @matches, "$match found at $hitpos ";
}

print "$_\n" foreach @matches;
print "$string\n";