Hello IB2017,

As fellow Monk haukex said, if the string that you want to compare have the same root e.g. NN or NNS (same root NN) then you can use:

#!/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!
[download]

Should be faster than grep you can Benchmark the codes.

Update: Removing unnecessary sample of code.

Hope this helps, BR.