Although these modules don't solve the problem you're asking directly, maybe they are a starting point: Regexp::Trie, Regexp::Assemble, Regex::PreSuf. Note that in your problem statement, using the a strings as an example, you're saying that you're looking for either 1, 2, 5, or 7 consecutive a's, but a+ actually matches more than that. Note how these optimizers are taking the number of a's into account.

use warnings;
use strict;

my @strs = qw/a b c aa bb ccc aaaaa bbb cccccc aaaaaaa bbbb ccccccc/;

use Regexp::Trie;
my $rt = Regexp::Trie->new;
$rt->add($_) for @strs;
print $rt->regexp, "\n";
# (?^:(?:a(?:a(?:aaa(?:aa)?)?)?|b(?:b(?:bb?)?)?|c(?:cc(?:cccc?)?)?))

use Regexp::Assemble;
my $ra = Regexp::Assemble->new;
$ra->add($_) for @strs;
print $ra->re, "\n";
# (?^:(?:a(?:a(?:(?:aa)?aaa)?)?|c(?:cc(?:c?ccc)?)?|b(?:b(?:b?b)?)?))

use Regex::PreSuf;
my $re = presuf(@strs);
print $re, "\n";
# (?:aa(?:aaa(?:aa)?)?|bb(?:bb|b)?|ccc(?:cccc?)?|[abc])
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