-- t2.pm --
package t2;

sub testing2 { print "Testing from t2.pm\n"; }
1;

-- t.pm -- 
package t;
use parent t2;

sub testing { print "Testing from t.pm\n"; }
1;

-- t.pl --
#!/usr/bin/perl
use t;

t::testing();
t::testing2();

##</code><code>##

Testing from t.pm
Undefined subroutine &t::testing2 called at ./t.pl line 5.