sub print_to_fh
{
  my ( $ref_to_fh ) = @_;
  #$ref_to_fh is _not_ a filehandle. It's a scalar, that's a reference.  
  my $filehandle = $$ref_to_fh;
  #Filehandle has dereferenced $ref_to_fh, so we can print to it now:
  print $filehandle "Some more text\n";
}

open ( my $filehandle, ">", "testfile.txt" );
&print_to_fh ( \$filehandle );
close ( $filehandle );