look at this code:

#!perl -w
use strict;
my $var = "111";
$var = undef;
print $var;
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The output is "Use of uninitialized value in print at script.pl line 5.". That is because $var is first set to "111" and then to undef afterwards. Likely it is that what happens in your script, e.g. a subroutine you call can return undef.