Start deleting lines of code until the warning disappears. I can not reproduce the warning with the code that you have posted.

Inspect %INC to make sure you are looking in all the modules you think you are looking in:

use Data::Dumper;
print Dumper(\%INC);
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Update: Or try this hack:

for my $file (values %INC) {
    open my $fh, '<', $file;
    while (<$fh>) {
        print "$file: $_" if /\bsub\s+greater\b/
    }
}
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Well, the code I provided was more of a "this is how I do it" in the case that I do it wrong, someone can point it out.

That's a good idea though; I didn't think of %INC.

I've checked out all of the included files in %INC, still nothing shows up. Any other suggestions?

Maybe Devel::Symdump can help you locate the duplicate subs:

use Data::Dumper;
use Devel::Symdump;
my $s = Devel::Symdump->new();
print Dumper($s->functions());
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I could post an actual subroutine, if someone might find it of some value...

Yes, let's see! As others have mentioned, your code doesn't have anything visibly wrong with it in isolation (although you probably want to call your subroutine using the modern syntax greater(@args) rather than &greater(@args)).

I think just as useful as the actual code snippet is the exact error message, wording, line numbers, and all.

Rather than posting an entire script (because I don't have a small example), here's another thought to mull over...

In one particular case, the error "subroutine X redefined at ..." appears only sometimes. I was able to duplicate it, not by refreshing my page, but by navigating back and reclicking the link.

Could this have something to do with a previously-loaded subroutine in my server process' memory that's being redefined?

I'm a little confused—you say “I could post an actual subroutine”, but then say that you don't want to post a code snippet?

Anyway, I think that it's important to realise that Perlmonks are happy to give you all the help we can, but that you have to show that you're willing to meet us halfway—saying “I won't take the effort to make a small code sample, but you should take the effort to guess why a problem might be occurring anyway” isn't likely to lead anywhere useful.

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;
use Devel::Symdump;
use Sub::Installer;

my $PackageName = 'mysub';
my $sub = \&greater;

$PackageName->reinstall_sub({$sub => \&greater});

my $s = Devel::Symdump->new();
print Dumper($s->functions());

sub greater {
  my ($arg1, $arg2) = @_;
  print $arg1 > $arg2 ? $arg1 : $arg2;
}
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