With slightly easier to read vars:

sub p{($a,$b,@_)=@_;@*=($b)x(-@_+abs$a);$a>0?(@_,@*):(@*,@_)}
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Also the prototype of abs makes this annoying, i thought you could reorder the -@_+abs$a to lose one of the operators but the abs() then requires parens. Nice one by the way. :-)