use strict;
use warnings;

my ($in, $out, %f1);

open($in, 'in1.dat');
while (<$in>) {
    chomp; $f1{$_} = ();
}
close($in);

open($out, '>out.dat');
open($in, 'in2.dat');
while (<$in>) {
    print $out if !exists $f1{(split /\t/)[0]};
}
close($in);
close($out);