The problem is that input is normally line buffered, but you could use Term::ReadKey.  Something like

#!/usr/bin/perl

use strict;
use warnings;
use Term::ReadKey;

$| = 1;

print "\e[21t";

ReadMode 3;
my $key;
while (not defined ($key = ReadKey(-1))) { }
while (defined ($key = ReadKey(-1))) {
    print "key: $key\n";
}
ReadMode 0;

__END__
key: ]
key: L
key: T
key: e
key: r
key: m
key: i
key: n
key: a
key: l
key: 
key: \
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