$page_data = "the cow jump over the moon";
$new = join(" ", grep { length($_) > 3} split(/\s/, $page_data));
print $new;
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See perlfunc manpages of split,join and grep for how this works, basically:

• 1. split makes an array of the original line
• 2. grep filters for elements longer than 3 chars
• 3. join glues them together