# a sample file, lets say a *really* long /etc/passwd file
# with lines like
root:x:0:0:Super User:/:/sbin/sh
someone:x:45:14:Some One:/home/someone:/bin/csh
sometwo:x:46:14:Some Two:/home/sometwo:/bin/ksh

# Lets say im gonna sort on uid then gid, I'll do
while (<IN>) {
   chomp;
   ($uid,$gid) = ( split(/:/) )[2,3];
   push(@tosort, [$uid, $gid, $_]);
}
...

for ( sort { $a->[0] <=> $b->[0] || $a->[1] <=> $b->[1] } @tosort ) {
   print "$_->[2]\n";
}