#! /usr/bin/env perl
use strict;
use warnings;

use 5.016;

# Position    0        10        20
#             |        |         |
my $string = "123abc789def345abc901";
my $search = "abc";
my $length = length($search);


while ( $string =~ m/$search/gi ) {
    say pos($string)-$length;
}

__END__