This is one way to handle this problem. It is tied to the exact layout of @matrix, so beware:

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


my @matrix = ([3, 4, 10, 2],
              [2, 7, 12, 3],
              [0, 3, 4, 5],
              [6, 5, 9, 11]);
      
print_2by2(\@matrix);

sub print_2by2 {
    my $aref = shift;
    my @copy = @{$aref};
    my $length = scalar @matrix;
    my $count = 1;
    while ( $length > 0 ) {
        $length -= 2;
        my $rowref1 = shift @copy;
        my $rowref2 = shift @copy;
        my $rowlength = scalar @{$rowref1};
        for ( my $j = 0; $j < $rowlength/2; $j++ ) {
           my $index = $j + $count;
           print $index, ":";
           print "          ";
        }
        print "\n";
        $count += $rowlength/2;
    rowprint($rowref1);
    rowprint($rowref2);
    print "\n";
    }
}

sub rowprint {
    my $rowref = shift;
    my $rowlength = scalar @{$rowref};
    for ( my $i = 0; $i < $rowlength; $i++ ) {
        print $$rowref[$i];
        if ( $i % 2 == 0 ) {
            print ", ";
        } 
        else {
            print "        ";
        }
    }
    print "\n";
}
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The output is:

C:\Code>perl fancy_print.pl
1:          2:
3, 4        10, 2
2, 7        12, 3

3:          4:
0, 3        4, 5
6, 5        9, 11
[download]