##### aa.pm #####
package aa;

sub import {
   print "import in $_[0] called from ". (caller(1))[3] ." \n";
}

use bb;

# ...

1;


##### bb.pm #####
package bb;

sub import {
   print "import in $_[0] called from ". (caller(1))[3] ." \n";
}

use aa;

# ...

1;


#################
>perl -e "use aa; use bb;"
import in aa called from bb::BEGIN
import in bb called from aa::BEGIN
import in aa called from main::BEGIN
import in bb called from main::BEGIN