comment on

Similarly, I've started noticing issues with the common practice of doing local $_. In some situations, that's not enough to prevent damage to the caller. Using local *_ fixes those problems. For example, consider the following function:

sub dequote {
   local $_ = @_ ? $_[0] : $_;
   s/^"//;
   s/"$//;
   s/\\(.)/$1/g;
   return $_;
}
[download]

It assigns its parameter (or $_ if there are no parameters) to $_. It attempts to prevent damage to the caller by localizing $_ first, but I will show it doesn't always work.

Case 1

$_ is tied, or $_ is an alias to something which is tied.

Running

use strict;
use warnings;

sub dequote {
   local $_ = @_ ? $_[0] : $_;
   s/^"//;
   s/"$//;
   s/\\(.)/$1/g;
   return $_;
}

{
   package MyTie;
   sub TIESCALAR { bless(\my $var, shift) }
   sub FETCH     { my $self = shift;
                   print("FETCH\n"); return $$self; }
   sub STORE     { my $self = shift; $$self = shift;
                   print("STORE $$self\n"); return $$self;}
}

tie my $var, 'MyTie';
$var = '"John \"Foo\" Bar"';

print dequote, "\n"
   foreach $var;
[download]

gives

STORE "John \"Foo\" Bar"
FETCH
STORE
FETCH
Use of uninitialized value in concatenation (.) or string at 561931.pl
+ line 18.
STORE
FETCH
Use of uninitialized value in substitution (s///) at 561931.pl line 6.
FETCH
Use of uninitialized value in substitution (s///) at 561931.pl line 7.
FETCH
Use of uninitialized value in substitution (s///) at 561931.pl line 8.
FETCH
STORE
Use of uninitialized value in print at 561931.pl line 25.
[download]

Case 2

pos($_) is used by the caller.

Running

use strict;
use warnings;

sub dequote {
   local $_ = @_ ? $_[0] : $_;
   s/^"//;
   s/"$//;
   s/\\(.)/$1/g;
   return $_;
}

$_ = 'abcd';
/\G .. /gcx;
print(pos(), "\n");

dequote('"John \"Foo\" Bar"');

print(pos(), "\n");
[download]

gives

2
Use of uninitialized value in print at 561931.pl line 18.
[download]

Changing
local $_ = @_ ? $_[0] : $_;
to
my $s = @_ ? $_[0] : $_;
local *_ = \$s;
gives

2
2
[download]

The Solution

sub dequote {
  #local $_ = @_ ? $_[0] : $_;   # XXX
   my $s = @_ ? $_[0] : $_;      # Fix
   local *_ = \$s;               # Fix
   s/^"//;
   s/"$//;
   s/\\(.)/$1/g;
   return $_;
}
[download]

After making that change, the programs output

STORE "John \"Foo\" Bar"
FETCH
John "Foo" Bar
[download]

and

2
2
[download]

respectively.

In reply to Re: Rethrowing with die $@ considered harmful (local $_ buggy) by ikegami
in thread Rethrowing with die $@ considered harmful by ysth

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