use strict;
use warnings;
use 5.010;

my $string = "CATINTHEHATWITHABAT";
my $regex = qr{\wAT}i;

while ($string =~ m/($regex)/g){
    my $match = $1;
    my $start = $-[0];
    my $end = $+[0];
    my $hitpos = "$start-$end";
    my $lcmatch = lc($match);
    $string =~ s/$match/$lcmatch/g;
    # in the next iteration start over where we left off
    pos($string) = $end;
    say "$match found at $hitpos ";
}

say "string: $string";