It's bad style, but yes:

package Blah;

sub doSomething
{
    $main::f->whatever();
}
[download]

As long as $f is a package variable in main -- i.e., not declared with my, my preference being:

our $f = Foobar->new();
[download]

... and as long as I don't have to maintain your code. You have factoring issues here that should be keeping you awake at night.