#!/usr/bin/perl 
use strict;
use warnings;   # the same as #!/usr/bin/perl -w in first line

while (<DATA>)
{
    next if /^\s*$/;     # skip blank lines
    chomp;               # removes trailing \n but not spaces
   
    my ($date) = split (/[,]/,$_);
    print "date_original       = $date\n",
          "date_sortable       = ", year_first($date), "\n",
          "original date back  = ", 
                        day_first(year_first($date)), "\n\n";
}

sub year_first          # convert 20/08/2007 to 2007-08-20
{
    my ($date) = @_;    # one way to get the sub's input value
    my @tokens = split (/\//, $date);
    @tokens = reverse @tokens;
    my $new_date = join('-',@tokens);
    return $new_date;
}

sub day_first          # convert 2007-08-20 to 20/08/2007
{
    my $date = shift;  # another way to get a single value
    my @tokens = split (/-/, $date);
    @tokens = reverse @tokens;
    my $normal_date = join('/',@tokens);
    return $normal_date;
}

=prints
date_original       = 20/08/2007
date_sortable       = 2007-08-20
original date back  = 20/08/2007

date_original       = 20/08/2008
date_sortable       = 2008-08-20
original date back  = 20/08/2008

date_original       = 04/04/2007
date_sortable       = 2007-04-04
original date back  = 04/04/2007

=cut

__DATA__
20/08/2007,Erythrocyte sedimentation rate,,3 mm/h
20/08/2008,Total white blood count,,6.7 10*9/L
04/04/2007,Haemoglobin estimation 12.9 g/dL,,12.9 g/dL