my $header;

# Handle the first file separately so we can keep the "categories."
my $file = shift;
$header = "CATS \t$file"; # CATS is category column header.
open my $fh, '<', $file or die "$file: $!";
my @lines = <$fh>;
close $fh;
chomp(@lines);

for my $file (@ARGV) {
    # Add filename to header.
    $header .= "\t$file";

    open my $fh, '<', $file or die "$file: $!";

    # Iterate through file
    my $i = 0;
    while (my $line = <$fh>) {
        chomp $line;
        # Append tab and 2nd column to appropriate line.
        $lines[$i++] .= "\t" . (split /\t/, $line)[1];
    }
    close $fh;
}
# print our header and each of our new lines.
print $header, "\n";
print "$_\n" for @lines;