How about this

use strict;
use warnings;
use 5.010;


my $string = "CATATHAT";
my $regex = '\wAT';

say 0..9;   #print ruler
say $string, "\n";  #print original string

while ($string =~ /$regex/gi) {
    my $match_len = length($&);
    my $start = length($`);
    my $end = $start + $match_len - 1;

    say "match: $& ", "start: $start ", "end: $end";
}

--output:--
0123456789
CATATHAT

match: CAT start: 0 end: 2
match: HAT start: 5 end: 7
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