Use of uninitialized value in range (or flip)
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$ perl -w -e 'my $x = 1..3'
Use of uninitialized value in range (or flip) at -e line 1.
$
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$ perl -w -e 'my $x = 1..3' -Mdiagnostics
$
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Furthermore, if diagnostics is used in conjunction with

use warnings FATAL => 'all';
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the script will not die because of the warning.

Workaround

We can't use diagnostics to get more information on this warning, but we can still use splain to do the job:

$ echo 'Use of uninitialized value in range (or flip)' | splain
Use of uninitialized value in range (or flip) (#1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl tells you what ope
+ration
    you used the undefined value in.  Note, however, that perl optimiz
+es your
    program and the operation displayed in the warning may not necessa
+rily
    appear literally in your program.  For example, "that $foo" is
    usually optimized into "that " . $foo, and the warning will refer 
+to
    the concatenation (.) operator, even though there is no . in your
    program.
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Well, this message is not all that helpful because it does not seem specific to this case. That's a separate issue for perldiag, which can be dealt with on another day.

Root cause

Here is my hypothesis on why this happens. A quote from Range Operator:

If either operand of scalar ".." is a constant expression, that operand is considered true if it is equal ("==") to the current input line number (the $. variable).

By default, $. is undefined:

$ perl -w -e 'print ">>>$.<<<\n"'
Use of uninitialized value in concatenation (.) or string at -e line 1
+.
>>><<<
$
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Unless you use diagnostics:

$ perl -w -e 'print ">>>$.<<<\n"' -Mdiagnostics
>>>0<<<
$
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So, it seems that diagnostics has the side effect of initializing the $. variable. I see some calls to open in the source code. Perhaps diagnostics is opening a file handle which is giving the global $. variable a value, thereby suppressing the warning message.

Possible solution

If I modify diagnostics.pm by adding the following line to localize $., the problem is fixed:

local $.;
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--- diagnostics.pm      2009-06-15 04:21:42.000000000 -0400
+++ diagnostics.pm.fix  2009-12-05 10:29:53.096561000 -0500
@@ -222,6 +222,7 @@

 local $| = 1;
 my $_;
+local $.;

 my $standalone;
 my(%HTML_2_Troff, %HTML_2_Latin_1, %HTML_2_ASCII_7);
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Version

$ perl -v

This is perl, v5.10.0 built for x86_64-linux
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The $VERSION of diagnostics.pm is 1.17. The latest version I can find on CPAN is 1.18, and the issue exists there too.

Acknowledgments

In trying to unravel a range operator mystery, a Super Search led me to an observation by QM. I could not see the warning message because I had use diagnostics; in my code.