my %seen;
open my $FH, "<", "file1.txt" or die "cannot open ...";
while (<$FH>) {
    chomp;
    $seen($_) = 1;
}
close $FH;
for my $other_file (qw/ file2.txt file3.txt file4.txt .../) {
    open my $FH, "<", $other_file or die "cannot open ...";
    while (<$FH>) {
        chomp;
        if (exists $seen{$_}) {
            $seen{$_}++;
        }
    }
    close $FH;
}