Looking for a _ is probably the wrong way to decide this, as others have said. But here is how you get a sub name from a coderef:

sub getsubname {
    use B 'svref_2object';
    my $coderef = shift;
    join '::', eval { svref_2object($coderef)->GV->STASH->NAME },
               eval { svref_2object($coderef)->GV->NAME }
}
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Returns false if something goes wrong (like not getting a coderef). Name will be pkg::__ANON__ for anonymous subs. And if the coderef is to an exported sub, the original name and package will be given.