This attempt uses regular expression code blocks to set the format string. I was hoping to find a way to use regex conditional patterns but couldn't get it to work. Here it is

use strict;
use warnings;

my $fmt;

print
   map
   {
      sprintf qq{Date: %-12sFormat: %-s\n},
         $_->[0],
         $_->[1]
   }
   map
   {
       $fmt = q{Date not valid};
       m{(?x)
          ^\d\d(\D)\d\d(\D)\d{4}$
             (?{$fmt = qq{DD${1}MM${2}YYYY}})
       };
       m{(?x)
          ^\d{4}(\D)\d\d(\D)\d\d$
             (?{$fmt = qq{YYYY${1}MM${2}DD}})
       };
       m{(?x)
          ^\d\d(\D)\d\d(\D)\d\d$
             (?{$fmt = qq{DD${1}MM${2}RR}})
       };
       [$_, $fmt];
   }
   map {chomp; $_;}
   <DATA>;

__END__
99/99/9999
99/99/99
99-99-9999
99-99-99
99-99
99-99-999
99:99:9999
99:99:99
99+99+9999
9999-99-99
9999:99:99
9/99/99
9999+99+99
99-99.9999
99.99-9999
[download]

and the output is

Date: 99/99/9999  Format: DD/MM/YYYY
Date: 99/99/99    Format: DD/MM/RR
Date: 99-99-9999  Format: DD-MM-YYYY
Date: 99-99-99    Format: DD-MM-RR
Date: 99-99       Format: Date not valid
Date: 99-99-999   Format: Date not valid
Date: 99:99:9999  Format: DD:MM:YYYY
Date: 99:99:99    Format: DD:MM:RR
Date: 99+99+9999  Format: DD+MM+YYYY
Date: 9999-99-99  Format: YYYY-MM-DD
Date: 9999:99:99  Format: YYYY:MM:DD
Date: 9/99/99     Format: Date not valid
Date: 9999+99+99  Format: YYYY+MM+DD
Date: 99-99.9999  Format: DD-MM.YYYY
Date: 99.99-9999  Format: DD.MM-YYYY
[download]

I hope this is of interest.

Cheers,

JohnGG

Update: Got the regex conditionals working so here's the script the way I originally intended. It produces identical output to the first version.

Update 2: Broke the closing brackets of the conditionals onto separate lines with appropriate indentation to aid clarity.

use strict;
use warnings;

my $fmt;

print
   map
   {
      sprintf qq{Date: %-12sFormat: %-s\n},
         $_->[0],
         $_->[1]
   }
   map
   {
       m
       {(?x)
          ^
          (?(?=\d\d(\D)\d\d(\D)\d{4}$)
             (?{$fmt = qq{DD${1}MM${2}YYYY}})
             |
             (?(?=\d{4}(\D)\d\d(\D)\d\d$)
                (?{$fmt = qq{YYYY${3}MM${4}DD}})
                |
                (?(?=\d\d(\D)\d\d(\D)\d\d$)
                   (?{$fmt = qq{DD${5}MM${6}RR}})
                   |
                   (?{$fmt = q{Date not valid}})
                )
             )
          )
       };
       [$_, $fmt];
   }
   map {chomp; $_;}
   <DATA>;

__END__
99/99/9999
99/99/99
99-99-9999
99-99-99
99-99
99-99-999
99:99:9999
99:99:99
99+99+9999
9999-99-99
9999:99:99
9/99/99
9999+99+99
99-99.9999
99.99-9999
[download]