As others have said, the code provides a default anonymous sub for the 'Post' attribute of $self. Since the anonymous sub aspect of this has been well covered, I'll comment on the default value side of things, with a slight detour into 'or' vs '||';

The code snippet you posted threw me off balance for half a second--setting default values is usually done using '||'--the high precedence logical or operator.

Perl has two logical or operators, '||' and 'or'. They identical except in terms of precedence. '||' has a high precedence and 'or' has a low precedence. 

$foo = $bar || 'baz';
# is identical to
$foo = ( $bar or 'baz');

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Common usage for each operator:

open( $fh, '>', $filename ) or die "Unable to open file: $!";

my $foo = $bar || 'baz';

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Now, lets look at a couple ways to set a default value: 

# Original code
$self->{'Post'} = ($args{'Post'} or sub {1;});

# High precedence or
$self->{'Post'} = $args{'Post'} || sub {1};

# Ternary operator
$self->{'Post'} = $args{'Post'} ? $args{'Post') : sub {1};

# Using '||' for a numeric value.
$self->{'Number'} = $args{'Number'} || 10;

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This idiom is a common, compact, and readable way to set default values. However there is a trap for the unwary here. If the value you are testing may legitimately evaluate to false, your default value will be spuriously assigned. Take a look at the code below for an example of this bug. 

foo();
foo(23);
foo(0);

sub foo {
    my $number = shift || 10;
    print "My number is $number\n";
}

__END__ #OUTPUT:
My number is 10
My number is 23
My number is 10

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So, if you use this idiom, make sure you understand how your acceptable values are treated in boolean context (whether they evaluate to TRUE or FALSE).

Update: as DrHyde points out, if you are using Perl 5.10, the truly lovely defined-or operator solves this problem very nicely. 

foo();
foo(23);
foo(0);

sub foo {
    my $number = shift // 10;
    print "My number is $number\n";
}

__END__ #OUTPUT:
My number is 10
My number is 23
My number is 0

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TGI says moo

In reply to Re: what is sub {1;} by TGI
in thread what is sub {1;} by mlgvalt

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