The bottleneck in your program seems the sorting at the end. Sorting is super-linear, which means that if your input size doubles, the running time more than doubles.
I'd write your program slightly different, but that shouldn't make much of a difference:
my (%hash, $line);
while ($line = <STDIN>) {
while (my ($word) = (lc $word) =~ /[a-z]{2,}/g) {
next if $word =~ /(.)\1\1\1\1/;
$hash{$word}++;
}
}
print "$_\n" for sort keys %hash;
Now, if the words are in order, there's no need for the hash, or the sorting:
my ($line, $prev);
$prev = "";
while ($line = <STDIN>) {
while (my ($word) = (lc $word) =~ /[a-z]{2,}/g) {
next if $word =~ /(.)\1\1\1\1/;
next if $word eq $prev;
print "$word\n";
$prev = $word;
}
}
(Code fragments are untested)