I think you don't need any extra array. Just the sub you already have. If some param is not passed, null will be assigned to $array->[x]:

C:\>perl -le "use strict;use warnings;sub f{my($x,$y)=@_;my $aa=$x;my 
+$bb=$y; print $aa, ' ', $bb}; f(1); f(2,3)"
Use of uninitialized value $bb in print at -e line 1.
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This means, that in first case $bb equals 'undef' as you want.