use strict;
use warnings;

##</code><code>##

...
my $math=<STDIN>;
...
my $sci=<STDIN>;
...

##</code><code>##

Bareword "n" not allowed while "strict subs" in use at ...

##</code><code>##

use strict;
use warnings;

my @subjects = qw( maths science history geography );
my $sum = 0;

foreach my $subject (@subjects)
{
    print "Enter percentage mark in $subject:\n";
    $sum += <STDIN>;
}

printf "Your average is: %.2f%%\n", ($sum / scalar(@subjects));