#!/usr/bin/perl

use strict;
use warnings;

sub print_to_fh
{
  my ( $ref_to_fh ) = @_;
  #$ref_to_fh is _not_ a filehandle. It's a scalar, that's a reference. 
  print $ref_to_fh "Some text\n";
}

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