Practical experimentation shows this doesn't work. :-(

use 5.010;
use Test::More;
use IO::Handle;

open my $fh, ">", "/tmp/foo" or die "argh";

ok $fh->can("close");
ok UNIVERSAL::can($fh, "close");

done_testing();

__END__
not ok 1
#   Failed test at fh.pl line 7.
not ok 2
#   Failed test at fh.pl line 8.
1..2
# Looks like you failed 2 tests of 2.
[download]

Not really surprising, since, according to perldoc,

UNIVERSAL is the base class from which all blessed references inherit

that is, the can method applies only to objects.

Would something like this do?

#! perl
use strict;
use warnings;

my $in_file  = 'temp1.txt';   # this must exist
my $outfile  = 'temp2.txt';
my $arrayref = [];

open(my $in,  '<', $in_file) or die "Cannot open file '$in_file' for r
+eading: $!";
open(my $out, '>', $outfile) or die "Cannot open file '$outfile' for w
+riting: $!";

can_write( $in_file,   $in);
can_write( $outfile,   $out);
can_write('$arrayref', $arrayref);

close($in)                   or die "Cannot close file '$in_file': $!"
+;
close($out)                  or die "Cannot close file '$outfile': $!"
+;

sub can_write
{
    my ($name, $fh) = @_;
    my  $result     = eval { no warnings; print $fh '' };

    printf "$name %s write\n", ($result ? 'can' : 'cannot');
}

__END__
temp1.txt cannot write
temp2.txt can write
$arrayref cannot write
[download]

Update: The CPAN module FileHandle::Fmode by Sisyphus/syphilis may also be worth investigating, although it too appears not to work with IO::All objects.

HTH,

Athanasius <°(((>< contra mundum

The strange thing is that although $fh->can('close') doesn't return true, it also doesn't complain about there being no can method, whereas $fh->might('close') does complain about a missing might method. So it is, in some manner being treated as an object that kinda half implements the UNIVERSAL interface.

Writing the empty string to a handle is quite a nice solution I suppose, but there may be some tied interfaces where printing even the empty string has side-effects. :-(

And as it happens I'm actually more interested in reading than writing.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'