I think with qr// your regex is compiled beforehand, so that it will run
faster, especially in a loop when you use the same regex over and over
again. This ?-xism is what you get when you print it. eg:
my $rex = qr/\d+/;
print $rex; # (?-xism:\d+)
Consider it as if you would do
my $sub = sub {};
print $sub; # CODE(0x1a6279c)
But why they just don't print REGEX:\d+, beats me ;-)
Regards,
Marc