Here's my take. Modifiying it to read input from a file or for pretty printing is left as an exercise.

#!/usr/bin/perl

use Data::Dumper;

my @students = qw {
  Thomas 60 10 20 85
  Samuel 35 90 80 65
  Adam   100 90 85 52
};

my %h;
while ( my ($student, @vals) = splice @students, 0, 5) {
  $h{$student} +=  @vals[$_] * (0.2, 0.25, 0.25, 0.3)[$_] for 0..3;
}

print Dumper \%h;
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Regarding the code provided by you: one hint - split / / is not probably what you want. Try split // (and see perldoc -f split). Another hint - there are at least three more obvious logical mistakes (...hash keys(mental confusion)...two issues with contents of @students (forgot about references, extra element, again confusion)...) .