A subroutine in Perl takes its parameters and flattens them into a single list, and implicitly puts them in an array called @_. $_[N] is the Nth element of the @_ parameters array.

The following simply assigns those elements to names:

sub do_something {
    my ($red, $yellow, $blue) = @_;
    ...
}
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To do what you want, you'll need to pass by reference, dereference the items within the sub and then act on them:

my ($red, $blue, $green) = qw(1 1 1);

sub do_something {
    my ($red, $blue, $green) = @_;
    
    $$red += 1;
    $$green += 30;
}

do_something(\$red, \$blue, \$green);

print "$red, $blue, $green\n";
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It may be easier for you to define a single hash with all of your globals instead of individual scalars, and just pass that around via reference:

my %hash = (
    red => 1, 
    blue => 1, 
    green => 1, 
);

sub do_something {
    my $href = shift;

    $href->{red} += 1;
    $href->{green} += 30;
}

do_something(\%hash);

print "$hash{red}, $hash{blue}, $hash{green}\n";
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Note that the longer the script gets, the harder it is to keep track of globals, especially when they get modified at-a-distance unexpectedly.

-stevieb