Why keep both %all_links and %distinct_links?   $links{$aLink}++; handles both cases. All you need is   keys %links to give you distinct links. And you don't need to iterate over hashs. All you need is something like 
foreach $link ( @extracted_links ) {
    $links{$link}++;

    $constrainted_linkes{$link}++
        if ( 0 < grep { $link =~ m/$_/ } @constraints;
}

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The rest is details.

In reply to Re: How do I...? - Looping on a growing hash by dws
in thread How do I...? - Looping on a growing hash by S_Shrum

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