can anyone explain why

You're putting a sub into a different package, but you're not in that package. If you want that, use something like this:

#!/usr/bin/perl -w
 
$var='one';
$mypackage::var='two';
{
    package mypackage;
    sub func {
        print "my \$var is $var\n";
    }
}
mypackage::func();
__END__
Name "main::var" used only once: possible typo at foo.pl line 3.
my $var is two
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