Silly me. When I posted this, I must have removed all of the pl2bat boilerplate considering it a distraction. But it wasn't pl2bat boilerplate; it was modified pl2bat stuff where the modifications were vital to the functioning of the script. The above code just doesn't work as-is. I guess nobody who bothered to try (if any) bothered to complain. Sorry about the mix-up. Below is a working version:

@rem = '--*-Perl-*--
@echo off
set > %tmp%/vienv.tmp
perl -x -S "%0" %TMP%/vienv.tmp
if NOT "%COMSPEC%" == "%SystemRoot%\system32\cmd.exe" goto endofperl
if %errorlevel% == 9009 echo You do not have Perl in your PATH.
if errorlevel 1 goto script_failed_so_exit_with_non_zero_val 2>nul
goto endofperl
@rem ';
#!/usr/bin/perl -w
#line 15
use strict;

Main( @ARGV );
exit( 0 );

sub Main {
    my( $file )= @_;

    my $start= tell(DATA)
        or  die "Can't tell(DATA): $!";
    open DATA, "+< $0"  or  die "Can't read self ($0): $!\n";
    seek( DATA, $start, 0 )
        or  die "Can't fseek(DATA,$start,0): $!";
    die "Expected :endofperl after __END__ of $0.\n"
        unless  <DATA> =~ /^\s*:endofperl\s*$/i;

    open TEMP, "< $file"
        or  die "Can't read $file: $!\n";
    chomp( my @curr= <TEMP> );
    close TEMP;
    my %curr= map { ( split /=/, $_, 2 )[0,1] } @curr;

    system( ( $ENV{VISUAL} || "notepad" ), qq("$file") );

    open TEMP, "< $file"
        or  die "Can't reread $file: $!\n";
    chomp( my @user= <TEMP> );
    close TEMP;
    my %user= map { ( split /=/, $_, 2 )[0,1] } @user;

    for my $key (  keys %user  ) {
        if(  ! exists $curr{$key}  ) {
            next;
        } elsif(  $curr{$key} eq $user{$key}  ) {
            delete $user{$key};
        }
        delete $curr{$key};
    }
    seek( DATA, 0, 1 )
        or  die "Can't fseek(DATA,0,1): $!";
    print DATA "echo on\n";
    for my $key (  keys %curr  ) {
        print DATA "set $key=\n";
    }
    for my $key (  keys %user  ) {
        print DATA qq{set $key=$user{$key}\n};
    }
    truncate DATA, tell(DATA);
}

__END__
:endofperl
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