$regex = qr/(this)is[a]test/i;
$regex = "$regex"; $regex = qr/$regex/;  # simulate freeze/thaw cycle
$regex = "$regex"; $regex = qr/$regex/;  # simulate freeze/thaw cycle
$regex = "$regex"; $regex = qr/$regex/;  # simulate freeze/thaw cycle
print "regex:  qr{$regex}\n";
__OUTPUT__
regex:  qr{(?-xism:(?-xism:(?-xism:(?i-xsm:(this)is[a]test))))}