This syntax is used in combination with the fields and base pragmas to detect access to non-declared attributes at compile time.

It works like this (tested in 5.8.5):

#!/usr/bin/perl -w
BEGIN {
    print "Compile time\n";
}

package Foo;

use fields qw(some_attribute);

sub new {
    return fields::new(@_);
}

sub test {
    my Foo $self = shift;  # note the my Foo $self
    $self->{does_not_exist} = 1;
}

package main;

my Foo $foo = Foo->new();
print "run time\n";
$foo->test();

print "ok\n";
__OUTPUT__
Compile time
No such pseudo-hash field "does_not_exist" in variable $self of type F
+oo at test.pl line 16.
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and if we remove the Foo in my Foo $self at line 15 (in sub test {})

Compile time
run time
No such pseudo-hash field "does_not_exist" at test.pl line 16.
[download]

Programming Perl 3 and the warnings also mention pseudo-hashes, but since perl v5.8.x 5.9 they've been replaced by restricted hashes in the "fields" pragma.

updated: fields.pm still uses pseudohashes in perl 5.8.x