You can do it at this way:

my $date = '23/06/2003';

$date =~ m!([0-9]{2})/([0-9]{2})/([0-9]{4})!;
[download]

or

 
$date =~ m!(\d{2})/(\d{2})/(\d{4})!;

print "$3-$2-$1\n";

__DATE__

2003-06-23
[download]

Updated!

my $date = '23/06/2003';
$date =~ s!(\d{2})/(\d{2})/(\d{4})!$3-$2-$1!;
print $date;

__DATE__

2003-06-23
[download]

      
--------------------------------
SV* sv_bless(SV* sv, HV* stash);