Sure, you can interpolate there. 

my $num_zeds = "13";
/z{$num_zeds}/

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But you don't want interpolation. You want something like the following: 

/(a+)(??{ 'x{'.length($1).'}' })/

[download]

In reply to Re: interpolating code in regex brace quantifier by ikegami
in thread interpolating code in regex brace quantifier by grobber

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