As I understand it you have several layers of delimiters. One idea for reusable code would be something that converts this into a multi-dimensional array -- one dimension per delimiter. E.g: 
my $aref = &fieldParse($fullString, '!!', ';', ',');

sub fieldParse {
    my $source = shift;
    my $ret = [];
    my $delim = shift;
    defined($delim) or return $source;
    for (split $delim, $source) {
        push @$ret, &fieldParse($_, @_);
    }
    return $ret;
}

[download]
which produces:- 
$VAR1 = [
          [
            [
              'abcd-efgh-ijkl-mnop'
            ],
            [
              'key1=data1',
              'key2=data2'
            ],
            [
              'key1=data
+3',
              'key2=data4'
            ]
          ],
          [
            [
              'qwer-asdf-zxcv-tyui'
            ],
            [
              'key1=data3'
            ],
            [
              'key3=data6'
            ]
          ],
          [
            [
              'trew-hgfd-yt
+re-bvcx'
            ],
            [
              'key1=data7',
              'key2=data8'
            ],
            [
              'key1=data9',
              'key2=data10'
            ]
          ],
          [
            [
              'erty-dfgh-cvbn-
+hjkl'
            ],
            [
              'key2=data5'
            ],
            [
              'key3=data6'
            ]
          ]
        ];

[download]
Updated (handle case of false-value delimiter as someone suggested)

One world, one people

In reply to Re: nth field extraction by anonymized user 468275
in thread nth field extraction by lee_crites

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