Or do: 
use Scalar::Util 'blessed';
my $answer = blessed($fd) && $fd->isa(...);

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which is better in that it allows you to stipulate that $fd is an object, not a class name.
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In reply to Re^3: Do not use UNIVERSAL::isa this way; but why? by ysth
in thread Do not use UNIVERSAL::isa this way; but why? by klekker

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