my $str   = "abcabcdefgabcdXfgabYdefgabcdZZg";
my $match = "abcdefg";
my $len = length($match);

for my $i (0..length($str)-$len) {
    my $s = substr $str, $i, $len;
    my $x = $s ^ $match;
    $x =~ tr/\0//d;  # equal characters xor to \0
    print "'$s' at offset $i matched" unless length($x) > 1;
}

__END__
'abcdefg' at offset 3 matched
'abcdXfg' at offset 10 matched
'abYdefg' at offset 17 matched