(a-z_) should probably be ([a-z_]*), or better yet, (\w*).

You're asking for trouble by putting your pattern in a string before using it in a regex. To avoid multiple-quoting headaches, use qr like this:

my $regexp = qr{(?:ftp:\/\/)?\/{5}(a-z_)\/};
if ($link =~ /$regexp/) { ... }
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