There are at least two reasons that I can think of..
- caller:
sub whoami {
print +(caller 1)[3], $/
}
+
*main::foo = sub { whoami() };
+
foo();
__END__
main::__ANON__
- DProf:
*main::foo = sub { sleep 1; };
+
foo() for 1..5;
__END__
$ perl -d:DProf foo.pl
$ dprofpp tmon.out
Total Elapsed Time = 5.00996 Seconds
User+System Time = 0 Seconds
Exclusive Times
%Time ExclSec CumulS #Calls sec/call Csec/c Name
0.00 - -0.000 5 - - main::__ANON__
When you have a lot of different anonymous subs floating around, it's a pain in the butt to profile, because all calls to anonymous subs land in main::__ANON__ as far as DProf is concerned.
There's a trick involving local *__ANON__, which works for caller but not DProf.