#!/usr/bin/perl -w
use strict;

my @strings = ("CATINTHEHATWITHABAT",
               "WERWERWAT134wAt");
               
my $regex = '\wAT';

foreach my $string (@strings)
{
   while ( $string =~ m/($regex)/gi)
   {
       printf "%s found at pos %2d in %s\n",
               $1, pos($string)-length($1)+1, $string;
   }
}

__END__
Prints:
CAT found at pos  1 in CATINTHEHATWITHABAT
HAT found at pos  9 in CATINTHEHATWITHABAT
BAT found at pos 17 in CATINTHEHATWITHABAT
WAT found at pos  7 in WERWERWAT134wAt
wAt found at pos 13 in WERWERWAT134wAt