I went with a split, hash, and sort method.

#!/usr/bin/perl -w
# slurped file in @numbers
# final outcome in %numHash

@numbers = ( "432/205", "905/4578", "623/4568", "1/10009", "10/10005" 
+);
foreach ( @numbers ) {
  ($key, $value) = split /\//, $_;
  $numHash{$key} = $value;
}

# UPDATED
foreach $numerator ( sort { $a <=> $b } keys %numHash ) {
  print $numerator , '/' , $numHash{$numerator} , "\n";
}
[download]

TIMTOWTDI
Dex

p.s. - mine won't handle leading zeros, but the pizza man is en route!

UPDATE 0: - forgot my <=>!!!!! Need fud!

UPDATE 1: - check out how do i sort hash by key numerically