Fix up and refactor util/perl-nbsp.p6

Sometimes the script would replace the "6" in "Perl 6" with nothing for
whatever reason. That no longer happens. A filename can now be passed to
the script so it'll only check that file for any instances of "Perl 6"
using regular spaces.

util/perl-nbsp.p6

@@ -1,68 +1,63 @@
 #!/usr/bin/env perl6
-
 use v6;
 use lib 'lib';
 use Test-Files;
 
-my $degree = %*ENV<TEST_THREADS> // 2;
-my @files = Test-Files.documents;
+enum Syntax <CodeDoc TextDoc>;
 
-enum Syntax (
-    CodeDoc => 0,
-    TextDoc => 1
-);
+my $degree = %*ENV<UTIL_THREADS> // 2;
 
-sub check-line($line, $state) {
+sub check-line(Str $line, Syntax $state) {
     given $line {
-        when /^\=begin\scode/ { CodeDoc }
-        when /^\=for\scode/   { CodeDoc }
-        when /^\=end\scode/   { TextDoc }
-        when /^\=\w+/         { TextDoc }
-        when /^\s ** 4/       { CodeDoc }
-        default               { $state  }
+        when / ^ '=begin code' / { CodeDoc }
+        when / ^ '=for code' /   { CodeDoc }
+        when / ^ '=end code' /   { TextDoc }
+        when / ^ '=' \w+ /       { TextDoc }
+        when / ^ \s ** 4 /       { CodeDoc }
+        default                  { $state  }
     }
 }
 
-my @promises = @files.map(-> $file {
-    Promise.start({
-        my Str    @in       = $file.IO.lines;
-        my Str    @out      = [];
-        my Syntax $state    = TextDoc;
-        my Bool   $modified = False;
-        my Bool   $split    = False;
-        for @in -> $in-line {
-            unless $in-line {
-                @out.push($in-line);
-                next;
-            }
-
-            $state = check-line($in-line, $state);
-            if $state ~~ CodeDoc {
-                # Perl 5 and Perl 6 should keep regular spaces in code.
-                @out.push($in-line);
-                next;
-            }
-
-            my $out-line = $in-line;
-            if $split {
-                $out-line = $in-line.subst(/^\s?<?before 5||6>/, "\x00A0");
-                $split = False;
-            } else {
-                $out-line = $in-line;
-                $split = True if $out-line.ends-with('Perl');
-            }
-
-            $out-line ~~ s:g/Perl\x[0020](5||6)/Perl\x[00A0]$0/;
-            $modified = True if $out-line ne $in-line;
-            @out.push($out-line);
+multi sub replace-spaces(Str $file) {
+    nextwith $file.IO;
+}
+multi sub replace-spaces(IO::Path $file) {
+    my Syntax $state    = TextDoc;
+    my Bool   $modified = False;
+    my Bool   $split    = False;
+    my Str    @in       = $file.lines;
+    my Str    @out      = @in.hyper(:$degree).map(anon sub (Str $in-line) {
+        return $in-line unless $in-line;
+
+        $state = check-line $in-line, $state;
+        # Perl 5 and Perl 6 should keep regular spaces in code.
+        return $in-line if $state == CodeDoc;
+
+        my Str $out-line = $in-line.clone;
+        if $split {
+            $out-line ~~ s/ ^ \x[0020]? <?before 6 | 5> /\x[00A0]/;
+            $split = False;
+        } else {
+            $out-line ~~ s/ 'Perl' [ \x[0020]+ | \x[00A0] ]? $ /Perl/;
+            $split = True if $/;
         }
 
-        if $modified {
-            say "Corrected mentions of Perl 6 to use NBSP in '$file'.";
-            $file.IO.spurt(@out.join("\n"), :close);
-            $modified = False;
-        }
-    })
-});
+        $out-line ~~ s:g/ 'Perl' \x[0020] ( 6 | 5 ) /Perl\x[00A0]$0/;
+        $modified = True if $out-line ne $in-line;
+        $out-line
+    });
 
-@promises.race(:$degree);
+    if $modified {
+        say "Corrected mentions of Perl 5 and 6 to use NBSP in '$file'.";
+        $file.spurt(@out.join("\n"), :close);
+    }
+}
+
+multi sub MAIN() {
+    Test-Files.documents.race(:$degree).map(&replace-spaces);
+}
+multi sub MAIN(Str $file) {
+    die "$file does not exist!" unless $file.IO.e;
+    die "$file is a directory!" if $file.IO.d;
+    replace-spaces $file.IO;
+}