TIP: Use local when assigning to $_

[Randy W. Sims]

[I see this a lot on these groups. I know most of the time it is just 
used for examples, but I thought I would point out that it is a bad habit.]

Anytime you make a direct assignment to the special variable $_, you 
should local-ize it. This code demonstrates the problem:

#!/usr/bin/perl
use strict;
use warnings;
my @array = (1..5);
sub nl {
    $_ = "\n";
    print;
}
foreach (@array) {
    nl;
    print;
}
__END__
The foreach assigns each element of @array to the special variable $_. 
Then the function nl() is called. The first thing it does is assign a 
value to $_ which clobbers the previous value. That value is then 
printed as the implied default argument to print. Control returns from 
the function and print is called again with the implied default value in 
$_ which is still the value assigned in nl rather than the value 
assigned by foreach.

Localizing by changing the assignment in nl() to:
local $_ = "\n";
results in the correct output. The assignment is made local to the 
enclosing scope, which in this case is the nl() function. When that 
scope is entered the old value of $_ is saved before any assignment is 
made. Then when the scoped is exited, the old value is restored.

Randy.
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