Did you have something like this in mind?

#!/usr/bin/perl
use strict;
use warnings;
our $var = 'foo';
sub test {
    my $var = 'bar';
    print $main::var, "\n";
    print "$var\n";
}
test();
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Output:

foo
bar
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Notes:

1. You really shouldn't reuse variable names like this. As you have already discovered it makes debugging a lot more difficult and increases the likelihood of making mistakes.
2. You should read up on 'our' to understand the full ramifications of using it (e.g. run perldoc -f our).