my $g=<<'_END_';
## START

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

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

# comment !
#         @
#         #
key3=val3
_END_


my $id = 'key3';
my ($ret) = $g =~ m/(.*)(?!\n\s*\n)\n$id/s;

print $ret;