It's generated by code something like these:

$ perl -wE 'my $vals; my $y = index($vals, "x")'
Use of uninitialized value $vals in index at -e line 1.
$ perl -wE 'my $vals; my $y = index("x", $vals)'
Use of uninitialized value $vals in index at -e line 1.
[download]

Look in /homedir/perl5/lib/perl5/CGI.pm, go to line 1243, and start investigating from there.

I do not have code calling the method index, but rindex yes.

What version of CGI do you have ?. In 3.65 the code was

sub STORE {
    my $self = shift;
    my $tag  = shift;
    my $vals = shift;
    my @vals = index($vals,"\0")!=-1 ? split("\0",$vals) : $vals;
    $self->param(-name=>$tag,-value=>\@vals);
}
[download]

In 4.37 it is

sub STORE {
    my $self = shift;
    my $tag  = shift;
    my $vals = shift;
    my @vals = defined($vals) && index($vals,"\0")!=-1 ? split("\0",$v
+als) : $vals;
    $self->param(-name=>$tag,-value=>\@vals);
}
[download]

poj

Something like this will do it (note the "-w" which I suppose you are using too):

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

CGI->STORE ('foo', undef);
[download]

So you can trigger this by passing an undefined parameter value to STORE() but only with "-w". Use warnings instead if you don't want to see it.

I do not have any CGI code calling CGI->STORE. Perhaps something else that calls CGI->STORE indirectly? What could it be?

While you could certainly investigate and debug CGI.pm, you really shouldn't use -w on the shebang line (#!/usr/bin/perl -w) and use warnings; instead. See What's wrong with -w and $^W.

use warnings;
my %hash;
if ($hash{'word'} =~ /world/) {
}
[download]

Saw this in my web log file today: $vals is not initialised, yes, but what precondition caused the warning to be triggered?

Precondition?

Some code you wrote somewhere? Some warnings you turned on for code you did not write? Don't turn on global warnings? Dont use old versions of CGI.pm? Be better programmer?