package MyTest;

use strict; 
use warnings;

sub findsub {
    
    my $subref;
    
    if( defined &main::testsub ) {
        $subref = \&main::testsub;
    } else {
        die "main::testsub doesn't exist.\n";
    }
    $subref->( "Hello world!\n" );
}

1;

package main;

MyTest::findsub();


sub testsub {
    print shift;
}