Date::Calc has a check_date() method.

use Date::Calc qw/ Delta_Days check_date /;

...
while( ... ){
  ...
  my ($line_year, $line_mon, $line_day) = /(\d{4})-(\d{1,2})-(\d{1,2})
+/
    or next    #  <==== this is probably desired as well, though check
+_date will catch (undef,undef,undef)
  ; 
  if( ! check_date( $line_year,$line_mon,$line_day ) ){
    warn "Bad date";
    next;
  }
  my $delta_days = Delta_Days($line_year,$line_mon,$line_day,$yearto,$
+monthto,$dayto);
  ...
}
[download]