use warnings;
use strict;
use Data::Dumper;

my @Definition=("NN", "NNS");
my ($defs) = map { qr/$_/ } join '|', map {quotemeta}
	sort { length $b <=> length $a } @Definition;

for my $str ( "ABC", "NXY", "NNF", "NNSX" ) {
	print "$str: ", $str=~/^$defs/ ? "yes!\n" : "no\n";
}

__END__

ABC: no
NXY: no
NNF: yes!
NNSX: yes!