No idea what causes it either, but it's clearly a split issue, since @X isn't even needed, although as others point out a my @X fixes it. Warped! (Although come to think of it, not using @X just makes perl use @_, so this may not mean much...)

#!/usr/bin/perl
#
## To burn up memory quickly, run this with option "death".

$option = shift;
if ($option eq "death") {
    $line = "1::";
} else {
    $line = "1:";
}
my $i=1;

while (1) {
    split(/:/, $line);
    # this sucks up memory too if you replace that line with
    # these 2
    #@X = split(/:/, $line);
    #@X = ();
    if(!($i++%3000000)) {
        # a good place to break point
        print "$i\n";
    }
}
[download]

--
Mike