As the POD says, when you use ReadKey with 0 or higher, it does the equivalent of getc(3) which is buffered. You need to use non-buffered mode to do what you want. (Also when your program exits, you need to put the terminal back the way it was. I use an END block to allow for abnormal termination.)

Try this:

#!/usr/bin/perl
use strict;
use warnings;
use diagnostics;
use Term::ReadKey;

my $count = 0;
my $char = undef;

ReadMode('cbreak');

while (!defined $char) {
    $char = ReadKey(-1);
    $count++;
}

print "$count\n";

# Put the terminal back the way you found it.
END {
    ReadMode('restore');
}
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