for my $x ( 1 .. 10 ) {
    my @keys;
    open my $file_handle, '<', "$filename$x" or die $!;
    while( my $line = <$file_handle> ) {
        if( $x == 1 ) {
            print $header, "\n";
            @keys = split /\t/, $line;
        }
        else {
            # Do something with each line read in.
        }
    }
}