Any advice appreciated.

Ok, here goes.

     17   my $opentag = $config->{'moredata_opentag'} ? $config->{'mor
+edata_opentag'} : '';
     19   my $closetag = $config->{'moredata_closetag'} ? $config->{'m
+oredata_closetag'} : ''; # not required
     24     $dataname = defined($val->[0]) ? $val->[0] : '' ; #datanam
+e can be empty
     25     $format = $val->[1] ? ($val->[1]) : $format_cfg ;
     27     $dataname = $val ? $val : '';
[download]

Those are usually written as:

     17   my $opentag = $config->{ moredata_opentag } || '';
     19   my $closetag = $config->{ moredata_closetag } || ''; # not r
+equired
     24     $dataname = @$val ? $val->[0] : '' ; #dataname can be empt
+y
     25     $format = $val->[1] || $format_cfg ;
     27     $dataname = $val || '';
[download]

And:

     17   my $opentag = $config->{'moredata_opentag'} ? $config->{'mor
+edata_opentag'} : '';
     18   die "MoreData requires an open tag: $!" unless ($opentag);

     32   my $substrings_aref = _retrieve_strings($str, $opentag, $clo
+setag, $dataname);
     33   unless (scalar($substrings_aref)) { # no stringrefs returned
     34     die "cannot extract substrings from string: $!";
     35   }

     41   my $datasep_cfg = $config->{'moredata_datasep'};
     42   length($datasep_cfg) or die "The data separation string must
+ be configured: $!";
     43   my $hashsep_cfg = $config->{'moredata_hashsep'} || '';
     44   length($hashsep_cfg) or die "The hash separation string must
+ be configured: $!";
[download]

Would probably be better written as:

     17   my $opentag = $config->{ moredata_opentag } or die "MoreData
+ requires an open tag\n";

     32   my $substrings_aref = _retrieve_strings($str, $opentag, $clo
+setag, $dataname) or die "cannot extract substrings from string\n";

     41   my $datasep_cfg = $config->{ moredata_datasep } or die "The 
+data separation string must be configured\n";
     43   my $hashsep_cfg = $config->{ moredata_hashsep } or die "The 
+hash separation string must be configured\n";
[download]

     18   die "MoreData requires an open tag: $!" unless ($opentag);
     30   die "No format specified in args or configuration: $!" unles
+s $format;
     33   unless (scalar($substrings_aref)) { # no stringrefs returned
     34     die "cannot extract substrings from string: $!";
     35   }
     42   length($datasep_cfg) or die "The data separation string must
+ be configured: $!";
     44   length($hashsep_cfg) or die "The hash separation string must
+ be configured: $!";
     52   } else {
     53     die "no format : $!";
     54   }
[download]

The $! variable only contains useful information directly after the use of a function that accesses the system.    A mathematical, string or boolean expression has no effect on it.

$substrings_aref is a scalar variable and it is used in a boolean context so the use of scalar there is completely superfluous.

     72     die "Count of $count is not a pair of values in hash assig
+nment of values @array : $!" if ($count != (2 || 0));
[download]

Because 2 is a TRUE value the expression is just if $count != 2.    Or did you really want the expression: if $count != 2 || $count != 0?

     98   my @substrings = ($content, $datastring);
    102   return \@substrings unless ($stringlength);   
    105   return \@substrings if ($openposition == -1);
    121   @substrings = ($content, $datastring);
    122   return \@substrings if ($dataname eq '__data__'); # return c
+ontent and data as strings
    123   return \@substrings if ($dataname eq '__content__'); # retur
+n content and data as strings
    124   return \@substrings unless length($datastring); # no reason 
+to process empty datastring
    130     @substrings = ($content, '');
    131     return \@substrings;
    144   @substrings = ($content, $datastring);
    145   return \@substrings;
[download]

You don't need an array to return an array reference:

    102   return [$content, $datastring] unless ($stringlength);   
    105   return [$content, $datastring] if ($openposition == -1);

    122   return [$content, $datastring] if ($dataname eq '__data__');
+ # return content and data as strings
    123   return [$content, $datastring] if ($dataname eq '__content__
+'); # return content and data as strings
    124   return [$content, $datastring] unless length($datastring); #
+ no reason to process empty datastring

    131     return [$content, ''];

    145   return [$content, $datastring];
[download]

    148 sub _escape {
    149 my $separator = shift;
    150 my $regex = '\\' . join('\\', split(/''/, $separator) );
    151 return $regex;
    152 }
[download]

I am not sure what this subroutine is supposed to be doing but maybe you should have a look at the quotemeta function or the "\Q...\E" quotemeta escape sequence.    For example:

     64 sub _moredata_hash {
     65   my ($datastring, $datasep, $hashsep) = @_;
     68   my %hash;
     69   my @list = split /\s*\Q$datasep\E\s*/, $datastring;
     70   foreach my $item (@list) {
     71     my $count = my @array = split(/\s*\Q$hashsep\E\s*/, $item)
+;
     72     die "Count of $count is not a pair of values in hash assig
+nment of values @array : $!" if ($count != (2 || 0)); 
     73     $hash{$array[0]} = $array[1];
     74   }
     75   scalar %hash ? \%hash : undef;
     76 }
[download]