my variables don't come into existance until the end of the statement in which they are declared, which means that when the regex is being compiled, <update> the lexical scalar,<update> $regex doesn't yet exist.

Using our bypasses this problem.

If you want to avoid using a global, then pre-declare the lexical.

use strict;

my $regex;
$regex = qr /(              # Start capture
                \(              # Start with '(',
                (?:             # Followed by
                (?>[^()]+)      # Non-parenthesis
                |(??{ $regex }) # Or a balanced () block
                )*              # zero or more times
                \)              # Close capture
                )/x;            # Ending with ')'

my $text = '(outer(inner(most inner)))';

$text =~ /$regex/gs;

print "$1\n";

__END__
P:\test>junk2
(outer(inner(most inner)))
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Updated description in the light of liz's observation below.