#!/usr/bin/perl -w
use strict;

my @km_testlist = qw( 120 500 300 400);
my @LoH;  #this will be a list of references to hash tables

#now of course this just shows how to make and 
#sort this type of structure.
#the stuff in this loop goes into your line process loop..

foreach my $thiskm (@km_testlist) #building a test structure
{

   my %table = ();     #don't need the =() but makes purpose very clear.
   $table{'val1'}="something"; #this data doesn't matter
   $table{'val2'}="aaaa";
   $table{'km'}=$thiskm;

   push(@LoH,\%table); #pushes a ref to table hash onto @LoH
}

@LoH = sort {  my $a_km = $a->{'km'}; #don't need the $a or $b_km temp
               my $b_km = $b->{'km'}; #trying to be clear for you
                 $a_km <=> $b_km  
             }@LoH;

#so now instead of using the $vars, from each line, you use
#the values in the hash, instead of 'val1', I would give the
#sort of name you are already using.

foreach my $tab_hashref (@LoH)
{
   print "$tab_hashref->{'km'} $tab_hashref->{'val1'} $tab_hashref->{'val2'}\n";
}

#prints:
#120 something aaaa
#300 something aaaa
#400 something aaaa
#500 something aaaa