Z uses L, so L is compiled and executed before the body of Z is even parsed. At compilation of L the assignment to @EXPORT in package Z hasn't been executed, so tough noogies - L doesn't get sub zProc, since that isn't there yet... Exporter's import (which is what's executed in Z at Z->import() ) sees an empty @EXPORT.

Use a BEGIN block to execute the assignment before compiling L via use:

package Z;

use strict;
use Exporter;

BEGIN {
    our (@EXPORT) = qw(zProc subzProc);
    our (@ISA) = qw(Exporter);
}

use L;

sub zProc {
  print "I am zProc\n";
}

sub subzProc {
  print "I am subzProc\n";
  L::lProc();
  L::sublProc();
}
1;
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