use strict;

die "Usage:  $0  file.1  file.2\n"
    unless ( @ARGV == 2 );    # test for number of args

my %known;

open( F, "<", $ARGV[0] ) or die "$ARGV[0]: $!";
while (<F>) {
    chomp;
    $known{$_} = undef;  # don't need a value, just the key
}
close F;

open( F, "<", $ARGV[1] ) or die "$ARGV[1]: $!";
while (<F>) {
    my $key = ( split ' ', $_ )[0];  # get first "word"
    print unless exists( $known{$key} );
}
close F;

##</code><code>##

little_perl_script.pl  file.1  file.2  > target.set