use strict;
use warnings;

my $posfile = 'posfile.txt';
my $pos_two = 'pos_two.txt';
my @isos20  = qw/these are the array elements/;
my %isos20  = map { $_ => 1 } @isos20;

#open file to write filtered lines to
open my $OUT2, '>', $pos_two or die "cannot open $pos_two: $!";

#open file to filter
open my $IN2, '<', $posfile or die "cannot open $posfile: $!";

while (<$IN2>) {
	my $comp = ( split /,/ )[0];
	print $OUT2 $_ if $isos20{$comp};
}

close $IN2;
close $OUT2;