#!/usr/bin/perl

my $TEST = "foo";
if ( defined &{$TEST->test_sub} ) { }

__END__
Can't locate object method "test_sub" via package "foo" (perhaps you forgot to load "foo"?) at ./798474.pl line 4.

---

#!/usr/bin/perl

my $TEST = "foo";
if ( defined &{$TEST->test_sub} ) { }

package foo;

sub test_sub { print "hi\n" }
__END__
hi