Conditional my isn't allowed. Besides, your code doesn't compile. You want 
my %code = do { my $i = 0; map { $_ => $i++ } split /,/, $tokens };

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If you really want no explicitely declared temp variables, you can use the following: 

my %code = sub { map { $_[$_] => $_ } 0..$#_ }->( split /,/, $tokens )
+;

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But honestly, there's no need to limit the scope that badly. Either of the following are quite suitable: 

my $i = 0;
my %code = map { $_ => $i++ } split /,/, $tokens;

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my @code = split /,/, $tokens;
my %code = map { $code[$_] => $_ } 0..$#code;

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In reply to Re^3: Convert a string into a hash by ikegami
in thread Convert a string into a hash by vitoco

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