Script2:

#my $var = "xxx\n";
my $var="zzz\n" unless defined($var);
print $var;
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Name "main::var" used only once: possible typo at unlesstest.pl line 2
+.
zzz
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That happens because before doing the assignment, the right-hand-side of the statement must be resolved. So, a global variable $var is created, and then masked immediately by a lexical variable $var. These variables are not related, and the warning is about the global one, which isn't accessible any more, since it is masked immediately by the my variable.