What's happening is that "$x =~ /^(.{10)/" is matching the first ten characters, and copying them into $1 for future use. If you want to replace $x, you want one of

$x =~ s/^(.{10}).*/$1/;
($x) = $x =~ /^(.{10})/;
$x = substr($x, 0, 10);
susbstr($x, 10) = '';
[download]

Any of them work, so use whichever style you like most (personally, I think I prefer the last one).