You can mess with the internals : something like

use B qw/class svref_2object/; 
sub is_a_precompiled_regexp { 
    my $r = shift; 
    return 0 unless ref $r; 
    my $o = svref_2object $r; 
    return (class($o) eq "PVMG") && ($o->MAGIC->TYPE eq 'r') ? 1 : 0; 
}
[download]

Without looking at the internals, I don't have an answer.

Yves / DeMerphq
---
Writing a good benchmark isnt as easy as it might look.