You should either use

open my $fhout, '>', "./test_out.file" or die "$!";
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or

open (my $fhout, '>', "./test_out.file") || die "$!";
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. The code you posted is parsed as

open (my $fhout, '>', ("./test_out.file" || die "$!"));
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which is definitely not what you want.

To avoid the error you reported, though, use

my ($fhout) = @_;
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or

my $fhout = shift;
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Also note that the ampersand sign & is not needed when calling a sub (see perlsub).