Invoking the "Schwartzian Transform" does the magic:

use strict;
use warnings;

my %students;

$students{0354} = "Fred\t3.2";
$students{4873} = "Tom\t2.9";
$students{1522} = "Susan\t4.0";

my @GPASorted =
  map {$_->[0]}
  sort { $a->[1] <=> $b->[1] }
  map {[$_, ($students{$_} =~ /\t(\d*(\.\d*)?)/)[0]]}
  keys %students;

print "$students{$_}\n" for @GPASorted;
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Prints:

Tom    2.9
Fred    3.2
Susan    4.0
[download]