Fully qualify the name or import the items from one module into the other module that uses it.

package a;

sub a_test {
   print "Hello, world!\n";
}

package b;

&a_test;       # fails
&a::a_test;    # Hello, world!
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You can also use the Exporter stuff to export functions from one module into another that uses it. I won't go into details about this. Check out the perlmod documentation as well as the stuff for the Exporter module.