Both are possible; it all depends what you want to do. Here's a function that returns an open read handle:

sub rh {
  my $filename = shift;
  open my $in, '<', $filename
    or die "Couldn't open $filename for reading: $!\n";
  return $in;
}

my $in = rh( 'somefile.txt' );
print while <$in>;
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If your myfunction returns the contents of a file, then see ysth's reply.