Getopt::Long has a configration option called 'gnu_compat' which I thought might work, but...

gnu_compat

gnu_compat controls whether --opt= is allowed, and what it should do. Without gnu_compat, --opt= gives an error. With gnu_compat, --opt= will give option opt and empty value. This is the way GNU getopt_long() does it.

Note that --opt value is still accepted, even though GNU getopt_long() doesn't.

Your problem is that note at the end. You do not want "--opt value" to be accepted, you want it to behave like "--opt= nextarg".

In the source for Getopt::Long there is this code to handle gnu_compat:

# Check if there is an option argument available.
if ( $gnu_compat ) {
    my $optargtype = 0; # none, 1 = empty, 2 = nonempty, 3 = aux
    --cut--
    elsif ( defined $rest || @$argv > 0 ) {
        # GNU getopt_long() does not accept the (optional)
        # argument to be passed to the option without = sign.
        # We do, since not doing so breaks existing scripts.
        $optargtype = 3;
    }
    --cut--
    return (1, $opt, $ctl, $type eq 's' ? '' : 0)
        if $optargtype == 1;  # --foo=  -> return nothing
}
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I think if you set '$optargtype' to 1 here it would work.

Maybe Getopt::Long could add a configuration option called 'gnu_compat_strict' so you could have:

elsif ( defined $rest || @$argv > 0 ) {
    $optargtype = $gnu_compat_strict ? 1 : 3;
}
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