As Ikegami pointed out, sysread is unbuffered io and I see no reason to use it here. It's much easier to use the buffered io functions and here's a simple revision of your example code that does just that.

#!/usr/bin/perl
use strict;
use warnings;

my $fname = '/var/log/Xorg.0.log';

open my $fh, '<', $fname or die $!;

while ( my $line = <$fh> ) {
    print $line;
    sleep 1;
}
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