Without seeing your test data or the way you're calling sort_func, it's hard to tell exactly what's going on. I tried it with this setup:

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

my @array = (
  "row #1\t106\thello",
  "row #2\t101\tgoodbye",
  "row #3\t104\thi again!"
) ;
my $datafields = 3 ;
my $sort = 1 ;
my $sortby = "number" ;
print join ("\n", sort {&sort_func} @array) ;
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This appears to work for both cases you mentioned. I don't think it's a precedence bug either. My guess is that one of your multitude of control variables is fouled up (this seems especially likely for $sortby -- doing a case-sensitive comparison with "magic" multi-character literal strings is dangerous, especially with no default case to catch mistakes.) With that said, why use a separate subroutine at all?

my $col = 1 ; # this is called "$sort" and "$item_no" in the original
my @sorted = sort {(split ("\t", $b)) [$col] <=> (split ("\t", $a)) [$
+col]} @array ;
[download]

Like I said, I don't have your test data so that might be totally useless to you, but it does work with the test data I used above (of course your original worked for me too). HTH!