$ cat pm_1232441.pl
use strict;
use warnings;

sub xyz { print "I'm ::xyz()\n"; }

{
    package FOO;
    sub xyz { print "I'm FOO::xyz()\n"; }

    sub abc {
        print "I'm FOO::abc()\n";
        xyz();
    }
}

{
    package BAR;

    sub abc {
        print "I'm BAR::abc()\n";
        xyz();
    }
}

xyz();
FOO::xyz();
FOO::abc();
BAR::abc();


Roboticus@Waubli ~
$ perl pm_1232441.pl
I'm ::xyz()
I'm FOO::xyz()
I'm FOO::abc()
I'm FOO::xyz()
I'm BAR::abc()
Undefined subroutine &BAR::xyz called at pm_1232441.pl line 21.