It ain't pretty, but using the sexeger (reverse regex) technique, which can no doubt be implemented better, this works.

It requires you start with your lines in an unchomped array rather than a single line though.

my @g=<DATA>;

my $id = 'key3';

my $g = join'',reverse @g;

if (my ($ret) = $g =~ m/^$id=.*?\n(.*?)\n\n/s) {
    $ret = join$/,reverse split$/,$ret;
    print $ret;
} else {
    print 'Failed!'.$/;
}

__DATA__
## START

# comment 1
#     2
#     3
#     4
key1=val1

# info   a
#     b
#     c
#     d
#     e
key2=val2

# comment !
#     @
#     #
key3=val3
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