In Re: Validating a Code Reference I mention my favorite, Devel::Peek::CvGV(), which I just tested and using it on \&main::ImportedFunction appears to give you the original package name.
You could also try some of the other solutions mentioned in that thread.
Update: Now that I got first post (j/k), here is some tested code:
use Devel::Peek "CvGV";
my $x= CvGV(\&CvGV);
$x =~ s/^\*//;
$x =~ s/::[^:]+$//;
print "CvGV came from $x.\n";
which prints "*Devel::Peek::CvGV" then "CvGV came from Devel::Peek."
(but my friends call me "Tye")