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sub foo;

print( __PACKAGE__->can('foo') ?1:0,"\n");  # 1
print( defined(&foo)           ?1:0,"\n");  # 0
print( exists(&foo)            ?1:0,"\n");  # 1

my $method_ref = __PACKAGE__->can('foo');
print( $method_ref && defined(&$method_ref) ?1:0,"\n");