i rewrote your code a bit, but i don't see which format of hash you want. you should provide some (few!) input data and desired output. here's the code:

#!/usr/local/bin/perl -w
use strict;
use Fcntl ':flock';
my $file = "inputdata.txt";
open(INFILE, $file) or die "File Not Found: $!";
flock(INFILE, LOCK_EX);
my @students;
while (<INFILE>) {
   push @students, [split / /, $_];
}
my %hash;

# where's $grades coming from?
# $hash{$grades}= $students[0]*0.2 + $student[1]*0.25+$students[2]*0.2
+5 + $student
+s[3]*0.3;

# flock(INFILE, LOCK_UN); # don't use unlock, it's done automagically 
+by close()
close(INFILE);
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update: oops, i forgot 'my @students'

update2: ok, after you updated your code and added input and output, here's my suggestion:

use strict;
use Fcntl ':flock';
my $file = "inputdata.txt";
open(FH, $file) or die "File Not Found: $!";
flock(FH, LOCK_EX);
my @students;
my %hash;
my @mul = (0.2,0.25,0.25,0.3);
while (<FH>) {
  my ($student, @grades) = (split ' ', $_); # split at one or more whi
+tespaces
  my $grade = 0;
  for (0..$#mul) { $grade += $mul[$_] * $grades[$_] }
  $hash{$student} = $grade; # save the sum as a value to the student n
+ame
}
foreach my $key(sort {$hash{$b} <=> $hash{$a}} keys %hash){
  print "$key: $hash{$key}\n";
}
close(INFILE);
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