Presumably, his example a, b, and c arrays are datasets from some outside source, so your a+b suggestion likely doesn't add anything. Additionally, I don't even know what you meant to do with @where_to, as you're just assigning strings 'a', 'b' and 'c'.

The simplest way to do what the OP wanted would be as pjotrik suggested. Your recommendation of List::Util's shuffle is definitely a good suggestion though.

Addendum: A quick and dirty example...

#!/usr/bin/perl -w

use strict;
use List::Util 'shuffle';

my @array_a = (10, 22, 34, 21, 20, 12, 32, 31, 91, 20);
my @array_b = (10, 22, 34, 21, 20, 12, 32, 31, 91, 20, 11, 32, 44, 51,
+ 10, 22, 82, 71, 61, 54);
my @array_c = (14, 25, 64, 71, 80);

mass_shuffle(\@array_a, \@array_b, \@array_c);

sub mass_shuffle {
    # all sub-arrays passed by ref
    my @arrays = @_;

    # combine into one large array and shuffle it
    my @shuffler;
    foreach my $sub (@arrays) { push @shuffler, @{$sub}; }
    @shuffler = shuffle(@shuffler);

    # splice properly sized randomized replacements
    foreach my $sub (@arrays) {
        @{$sub} = splice @shuffler, 0, scalar(@{$sub});
    }
}
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