according to perldoc -f filehandle you can pass the filehandle to the subroutine

open my $sendmail, "|/path/to/sendmail -t $to" or die("couldn't open p
+ipe to sendmail");
print_details($sendmail);
[download]

BTW: you don't have to use the & to call a subroutine, and, IMVHO, should be aware of it's meaning before doing so.

regards,
tomte