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

sub check {
    return substr($_[0], 0, length($_[1])) eq $_[1];
}

foreach my $string ("ABC", "NXY", "NNF", "NNSX") {
    print "$string: ", check($string, "NN") ? "yes!\n" : "no\n";
}

__END__

$ perl test.pl
ABC: no
NXY: no
NNF: yes!
NNSX: yes!