Generate all combinations with restrictions

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Perl

algorithm

I have to fill 3 slots
slot1 can be filled with 'a' or 'b'
slot2 can be filled with 'x','y' or 'z'
slot3 can be filles with 'p','q', 'r' or 's'
 

So the valid combinations are
axp axq axr axs
ayp ayq ayr  ays
bxp bxq ... and so on


How can I generate all such combinations

What I have tried:

I have tried a perl code like

@p = qw (ab xyz pqrs )
foreach (@p) {
$l = ... ( I am not sure what to do next )
}