#!/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]!"
    }
}
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Update: Hash is a better data structure for such a task, though.

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

my %age_of = (John  => 20,
              Jane  => 19,
              Jack  => 21,
              Jules => 22,
              Jim   => 18);

my $whose_birthday = 'Jane';

$age_of{$whose_birthday}++;
say "Happy birthday $whose_birthday, you're now $age_of{$whose_birthda
+y}!";
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]