gistfile1.pl

use strict;
use feature 'say';
use CGI;

main() unless caller();

sub main {
    local $ENV{REQUEST_METHOD} = "GET";
    qstring_is_not_empty();
    qstring_is_empty();
    qstring_is_empty_string();
}

sub qstring_is_empty {
    say "qstring is EMPTY";
    local $ENV{QUERY_STRING};
    run()
}

sub qstring_is_empty_string {
    say "qstring is foo=''";
    local $ENV{QUERY_STRING} = "foo=";
    run()
}

sub qstring_is_not_empty {
    say "qstring is foo='bar'";
    local $ENV{QUERY_STRING} = "foo=bar";
    run()
}

sub run {
    CGI::initialize_globals(); # enable re-parsing
    my $q = CGI->new;
    if ($q->param('foo')) {
        say "foo evaluates to TRUE (@{[ $q->param('foo') ]})";
    } else {
        say "foo evaluates to FALSE";
    }
}

__END__

qstring is foo='bar'
foo evaluates to TRUE (bar)
qstring is EMPTY
foo evaluates to FALSE
qstring is foo=''
foo evaluates to FALSE