use IO::Handle;
my $fh = IO::Handle->new;
tie *$fh, ...;
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Update: Or the following slightly more low-level way, the relevant code of IO::Handle->new doesn't do a whole lot more than this:

use Symbol qw/gensym/;
my $fh = gensym;
tie *$fh, ...;
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Links: IO::Handle, Symbol.

Update 2017-09-30: One more, from here:

my $fh = \do { local *HANDLE };
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