N50 N90 calculation using Perl code

#/usr/bin/perl -w
use strict;
my ($len,$total)=(0,0);
my @x;
while(<>){
if(/^[\>\@]/){
if($len>0){
$total+=$len;
push @x,$len;
}
$len=0;
}
else{
s/\s//g;
$len+=length($_);
}
}
if ($len>0){
$total+=$len;
push @x,$len;
}
@x=sort{$b<=>$a} @x;
my ($count,$half)=(0,0);
for (my $j=0;$j<@x;$j++){
$count+=$x[$j];
if (($count>=$total/2)&&($half==0)){
print "N50: $x[$j]\n";
$half=$x[$j]
}elsif ($count>=$total*0.9){
print "N90: $x[$j]\n";
exit;
}
}