Foo{THING} notation is convenient for this,

my $glob = 'FOO';
if (defined do {
    no strict 'vars';
    *$glob{IO}}) {
    # ...
}
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or if you can hardcode the name of the glob,

if (defined *FOO{IO}) {
    # ...
}
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The foo{THING} is actually an IO::Handle object, a reference, so you can get by with

if (*FOO{IO}) {
    # ...
}
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