The symbol table is a hash, so you can check existence with exists. See perlmod for a description of symbol tables. 
use strict;
use warnings;
use 5.010;

{
    package Foo;
    our $bar;
}
{
    if (exists $::{'Foo::'}{bar}) {
        say "There's a Foo::bar with some sigil";
    }
    if (exists $::{'Foo::'}{baz}) {
        say "There's a Foo::baz with some sigil";
    }
}

[download]

(Update: added code)

In reply to Re: How to access globals in arbitrary classes, without using soft ref? by moritz
in thread How to access globals in arbitrary classes, without using soft ref? by llancet

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