A whole module that does what I want ... in some cases I'd want it to return true.

That is only the case if you define "what you want" as: "What my newly minted module does."

But, what does a.n.other user actually need? And does your is_filehandle() actually supply it?

That is, someone is writing a module that accepts a "file handle" as a input argument, and they want to know if what they've been given, is acceptable for their purpose. That means they can either read from it; write to it; or both. And your module fails to detect that information.

#! perl -slw
use strict;
use IO::Detect qw[ is_filehandle ];

sub funcTakesIO1 {
    my $fh = shift;

    if( is_filehandle( $fh ) ) {
        print $fh 'Bang!';
    }
    return;
}

sub funcTakesIO2 {
    my $fh = shift;

    if( is_filehandle( $fh ) ) {
        return <$fh>;
    }
    return;
}


eval {
    funcTakesIO1( \*STDIN );
} or warn "IO::Detect detected the wrong thing";

eval {
    funcTakesIO2( \*STDOUT );
} or warn "IO::Detect detected the wrong thing";

__END__
C:\test>junk5
Filehandle STDIN opened only for input at C:\test\junk5.pl line 9.
IO::Detect detected the wrong thing at C:\test\junk5.pl line 24.
Filehandle STDOUT opened only for output at C:\test\junk5.pl line 18.
IO::Detect detected the wrong thing at C:\test\junk5.pl line 28.
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What the user actually requires in that situation is something like siphylis' Filehandle::Fmode, which has been around for a few years and effectively renders your module redundant.