Here's a List::Util reduce solution that seg faults under 5.8.0 on Solaris, but works in ActiveState. :-)

use strict; 
use warnings;
use Data::Dumper;

my $params = [ 1, [ 2, 3, 4 ], 5 ];

use List::Util 'reduce';

sub multiplex {
    reduce {
        my $aref = ref($a) ? $a : [[$a]];
        my $bref = ref($b) ? $b : [$b];
        [
         map {
             my $b_item = $_;
             map [ @$_, $b_item], @$aref
         } @$bref
        ]       ;
    } @{shift()};
}

print Dumper(multiplex($params));
[download]

And here's the same algorithm without reduce (and without seg faulting):

sub multiplex {
    my $ret = [[]];
    for (@{shift()}) {
        my $bref = ref($_) ? $_ : [$_];
        $ret =
          [ map {
              my $b_item = $_;
              map [ @$_, $b_item], @$ret;
            } @$bref
          ];
    }
    $ret;
}
[download]