package Foo;
use strict;
use warnings;

sub foo { print "foo\n" }
sub bar {
  my $class = shift;
  if ( my $f = $class->can("foo") ) {
    print "yes, I can foo\n";
    $f->();
  }
}

package Bar;
use strict; use warnings;

our @ISA = "Foo";

sub foo;

package main;
use strict; use warnings;

Bar->bar();

# OUTPUT:
yes, I can foo
Undefined subroutine &Bar::foo called at ./test_foo line 17.

##</code><code>##

sub foo {}