$regex = join ...
$regex = qr/...

##</code><code>##

$regex_base_str = join ...
$regex_compiled = qr/...

##</code><code>##

# Simple case: OK - matches "a" or "b"
$ perl -E 'my $re = "a|b"; $re = qr{$re}; say $re'
(?^u:a|b)

# Complex case: NOT OK - matches "Xa" or "bY"
$ perl -E 'my $re = "a|b"; $re = qr{X${re}Y}; say $re'
(?^u:Xa|bY)

# Complex case: OK - matches "a" or "b" [fixed with "(?:...)"]
$ perl -E 'my $re = "(?:a|b)"; $re = qr{X${re}Y}; say $re'
(?^u:X(?:a|b)Y)