sub genFixedSubsets
{
    my( $size, @set )= @_;
    my @idx= reverse 0..$size-1;
    return sub {
        return   if  $size < @idx;
        my @ret= @set[@idx];
        my $i= 0;
        $i++  until  ++$idx[$i] < @set-$i  ||  $size < $i;
        $idx[$i]= 1+$idx[1+$i]   while  0 <= --$i;
        return @ret;
    };
}

my $gen= genFixedSubsets( $ARGV[0] || 3, 1..($ARGV[1]||5) );
my @subset;
while(  @subset= $gen->()  ) {
    print "@subset\n";
}
[download]

For example:

$ subsets 3 5
3 2 1
4 2 1
5 2 1
4 3 1
5 3 1
5 4 1
4 3 2
5 3 2
5 4 2
5 4 3
[download]