Your second example is close, but you want to call ReadKey with a -1 instead of a 0, to tell Term::ReadKey not to block and wait for a key.

What you may want to consider though, is that by running in a tight loop like this you are going to suck up CPU time in a big way. For your purposes (collecting random data presumably from keystroke timings), you would probably be better of intentionally blocking and just keeping track of the times between the keystrokes, which would be much less CPU intensive.

use strict;
use warnings;
use Term::ReadKey;
use Time::HiRes qw( gettimeofday );

ReadMode( 5 );

my $timings_needed = 15;
my @timings = ();

while ( $timings_needed-- ) {
    my $char = ReadKey(0);
    push( @timings, [ $char, gettimeofday() ] );
}

for my $time ( @timings ) {
    print "@{ $time }\n";
}

ReadMode( 0 );
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