sub compatible { 
    my ($s1, $s2) = @_; 
    $s1=~/\G_/gcs and $s2=~/./gcs or 
        $s1=~/./gcs and $s2=~/\G_/gcs or 
        return 0 until 
        $s1=~/\G\z/ or $s2=~/\G\z/; 
    1; } 
print(compatible("_8__3__19", "4_8___7__") ? "true\n" : "false\n");
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Update: this is wrong too, for the same reason as my other reply. Wait, I'll try to post a correction.

Update: Maybe this would work:

sub compatible {
    my ($s1, $s2) = @_;
    my $t;
    $s1=~/\G_/gcs and $s2=~/./gcs or
        $s1=~/(.)/gcs and $t = $1, $s2=~/\G_/gcs or $s2=~/(.)/gcs and 
+$t eq $1 or
        return 0 until
        $s1=~/\G\z/ or $s2=~/\G\z/;
    1;
}

printf "%s v %s ? %s\n", @$_, compatible( @$_ ) ? 1 : 0
        for [ qw[ _8__3__19 48____7__ ] ],  # compat
                [ qw[ _8__3__19 4_8___7__ ] ],  # compat
                [ qw[ _8__3__19 48_____7_ ] ];  # clash
[download]

Update: this is wrong too.

Update:

sub compatible {
    my ($s1, $s2) = @_;
    my($c1, $c2, $d);
    {
        $s1 =~ /(.)/gs or return 1;
        $c1 = $1;
        $s2 =~ /(.)/gs or return 1;
        $c2 = $1;
        if ($c1 eq "_") {
                if ($c2 eq "_")
                        { }
                else
                        { vec($d, ord $c2, 1)++ and return; }
        } else {
                if ($c2 eq "_")
                        { vec($d, ord $c1, 1)++ and return; }
                else
                        { $c1 ne $c2 and return; vec($d, ord $c2, 1)++
+ and return; }
        }
    redo;
    }
}

printf "%s v %s ? %s\n", @$_, compatible( @$_ ) || 0
        for [ qw[ _8__3__19 48____7__ ] ],  # compat
                [ qw[ _8__3__19 4_2___7__ ] ],  # compat
                [ qw[ _8__3__19 4_8___7__ ] ],  # clash
                [ qw[ __8_3__19 48____7__ ] ],  # clash
                [ qw[ __8_3__19 84____7__ ] ],  # clash
                [ qw[ _8__3__19 48_____7_ ] ];  # clash
[download]