sub comb {
    my $s = shift;
    if (@_ == 1) { print "$s$_\n" for @{$_[0]}; return; }
    comb("$s$_",@_) for @{shift()};
}
comb '',[qw(a b c)],[qw(1 2 3)],[qw(- + *)];

##</code><code>##

    if (@_ == 1) { dowhatever("$s$_") for @{$_[0]}; return; }