#!/usr/bin/perl
use strict;
use warnings;
use feature qw{ say };

my @student = qw( John Jane Jack Jules Jim );
my @age     =   (   20, 19,   21,   22, 18 );

my $whose_birthday = 'Jane';

for (my $i = 0; $i <= $#student; $i++) {
#                  ~~
    if ($student[$i] eq $whose_birthday) {
        $age[$i]++;
        say "Happy birthday $student[$i], you're now $age[$i]!"
    }
}