If you use the syntax

foreach (@date){

to access an array, Perl will automatically store the current value from the array in the magic variable $_, hence your assignment statement might look like:

$user_input->{start_date} = $_;

Alternatively, if you use the syntax

foreach $your_variable (@date){

the current array value is stored in $your_variable (or whatever scalar you put there). For more information, see Foreach Loops in perlsyn.