our is not in this case not be recognized as a subroutine call. The error indicates it's being used as a method call (in the indirect object syntax), but the value it's being called on, $show_fields, is not defined, and hence perl can't lookup the method to call. I would note that you'd probably should get an error from that suggests that it was being redeclared for some reason or that strict is not being used. (Since strict would give a different error on the use of an undeclared variable.)

package main;
my $var=6;
sub foo {
        our $var;
        $var=5;
}

print $var;
foo();
print $main::var;
print $var;
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What is means is, that within the subroutine foo, I can get to the package $var without having to use a full package name ($main::var). The our declaration makes it clear that within this lexical scope "$var" means the package variable, not the lexical $var that exists in the enclosing scope.

Sometimes there are uses for "our" inside of subroutines.

update: of course I meant $main::var