The comments explains what this piece of code is doing:

use B;

$foo = qr/foo/;
if ($foo->isa("Regexp")) {#check whether it is a Regexp object
    print "is a Regexp\n";
} else {
    print "is not a Regexp\n";
}

print B::svref_2object($foo)->MAGIC->TYPE;#if it is not a sv ref, perl
+ will not allow you to call svref_2object
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