Try this:

#!/usr/bin/perl
use strict;
use warnings;

my $x;
$x = $x+1;

print $x;
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I get:

Use of uninitialized value in addition (+) at tmp.pl line 6.
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Then try:

#!/usr/bin/perl
use strict;
use warnings;

my $x;
$x = $x ? $x+1 : 1;

print $x;
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to see the difference...

cLive ;-)