#!/usr/bin/perl
use strict;
use warnings;

my $number_of_cols_per_file =3;
my @data;                         #this is a 2-D Array

while (my $line = <DATA>)
{
   chomp $line;
   my (@cols) = split (':', $line);
   push @data, \@cols;
}

my $file_num=1;

while (@{$data[0]} > 1) # Any Columns after the name_column left?
{
    # generate the next file
    
    # This print would change to a "file open" statement for
    # file_num, n...
    
    print "File Number = ",$file_num++,"\n";
    
    foreach my $row_ref (@data)
    {
        my $row_name=$row_ref->[0];
        my @data_cols = splice (@$row_ref,1,$number_of_cols_per_file);
        print join(":", $row_name, @data_cols), "\n";
    }
}

=Prints:
File Number = 1
row_nameA:col2a:col3a:col4a
row_nameB:col2b:col3b:col4b
File Number = 2
row_nameA:col5a:col6a
row_nameB:col5b:col6b
=cut

__DATA__
row_nameA:col2a:col3a:col4a:col5a:col6a
row_nameB:col2b:col3b:col4b:col5b:col6b