It's simpler than that: The quantifier cannot be variable.

$ perl -E 'my $quant = 2; "ab" =~ /.{$quant}/ and say "Matched"'
Matched
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I didn't say you couldn't build regexps dynamically. I said the quantifier can't be variable. Perl regular expressions don't even have variables, so you couldn't possibly have shown one being used.

You can't have a quantifier until you have a regexp, and you don't have a regexp until you interpolate anything that needs to be interpolated.

What's the quantifier in the following?

$ perl -E 'my $x = "2"; "ab" =~ /.{$x}/ and say "Matched"'
Matched
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Are you saying it's different in this one?

$ perl -E 'my $x = "{2}"; "ab" =~ /.$x/ and say "Matched"'
Matched
[download]

What about this one?

$ perl -E 'my $x = "2}"; "ab" =~ /.{$x/ and say "Matched"'
Matched
[download]

In all cases, the quantifier is {2}. No variables is involved. Sure, the regexp is produced from a variable, but that has nothing to do with the quantifier.

Update: Clarified.

Perl regular expressions don't even have variables, so you couldn't possibly have shown one being used.

This is what I first thought you meant, but it didn't make any sense to me. If one distinguishes “regex variables” (variable at regex run-time) from “Perl variables” (constant at each regex run-time ^*), then, as you (basically) say, the only ones (I think) are the \1 back-references; but, when Hena said

I would assume that quantifier cannot be a '\x' variable

you said

It's simpler than that: The quantifier cannot be variable.

The only reason I could think of to make this comment is if there were some other sort of variable, but you seem to be saying that there isn't. Did I misunderstand?

UPDATE: ^* False, as I finally realised in Re^6: Regex fun.