The $hash{key} = $value || 0 construct avoids a value of undefined. It is like setting a default value, or if you are familiar with SQl, like the NVL() function. If you are going to test the value of a variable later in the program, and the value might be undefined, this is used to avoid "Use of uninitialized value" warnings.

use strict;
use warnings;
my (%hash, $value);

print "Without\n";

$hash{key} = $value;

if ($hash{key} == 42) {
  print "taking appropriate action\n"
}

print "With\n";

$hash{key} = $value || 0;

if ($hash{key} == 42) {
  print "taking appropriate action\n"
}
[download]

Output:

/wlsedi/howard$: perl pm.pl
Without
Use of uninitialized value in numeric eq (==) at pm.pl line 10.
With
/wlsedi/howard$:
[download]

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