I would like to call it a method too but where is the reference to the main class/package "object" as the first parameter when calling asub() (a-la $obj->asub() which sends $obj as the first parameter)?

Just write it down properly:

use strict;
use warnings;
sub asub { my $c = 0; print ++$c,": $_ - heee\n" for @_ }             
+                   
my $x = 'asub';
main->$x($x, 'blorf');
__END__
1: main - heee
2: asub - heee
3: blorf - heee
[download]

As you can see, the package - main - is passed into the sub as the first argument.

Note that the package passed is just a string, not a refenrence. It would be a reference, if the invocant was a blessed variable:

use strict;
use warnings;
sub asub { my $c = 0; print ++$c,": $_ - heee\n" for @_ }             
+                   
my $x = 'asub';
my $obj = bless \$x;
$obj->$x($x, 'blorf');
__END__
1: main=SCALAR(0x1492870) - heee
2: asub - heee
3: blorf - heee
[download]