...
while (<CG>)
{
    my $keyval = ( split )[3];
    for my $output ( keys %intervals )
    {
        if ( $keyval >= $intervals{$output}{'min'} and
             $keyval <= $intervals{$output}{'max'} )
        {
            open( my $ofh, '>>', $output ) or die "$out_path: $!\n";
            print $ofh $_;
        }
    }
}