Use pair. On *nix you could probably use fork instead of a thread:

#! perl -slw
use strict;
use threads;

pipe \*RLOG, \*WLOG;
my $oldfh = select(*WLOG); $| = 1; select($oldfh);

## pass the write handle WLOG to expect
async{ ## simulate expect
    my $i;
    while( sleep 1 ) {
        print WLOG "output line" . ++$i;
    }
};

open LOG, '>', 'nul' or die; ## The real logfile here

## read from the read handle RLOG, and write to the real log
while( <RLOG> ) {
    print; ## Inspect the output
    print LOG; ## and then log it for real
}
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