I like to validate the format as long as I'm re-formatting (note this does not actually completely validate the dates, see what happens to some invalid end of month dates below):

use Time::Piece;

my @have = ('1/2/2003', '2/31/2014', '4/31/2015', '4/5/2006', '10/11/2
+012');

for my $dt (@have) {
  my $d = eval { Time::Piece->strptime($dt, '%m/%d/%Y') } or warn "Inv
+alid date $dt\n";
  next unless $d;
  my $ymd = $d->ymd();
  print "$dt => $ymd\n";
}
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DateTime will validate the date more completely:

use strict;
use warnings;

use DateTime::Format::Strptime;

my $f = DateTime::Format::Strptime->new(
  pattern => '%m/%d/%Y',
);

my @have = ('1/2/2003', '2/32/2013', '4/31/2014', '4/5/2006', '10/11/2
+012');

for my $dt (@have) {
  my $d = $f->parse_datetime($dt) or warn "Invalid date $dt\n";
  next unless $d;
  my $ymd = $d->ymd();
  print "$dt => $ymd\n";
}
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