Should be portable, and probably quicker than most other methods:

#! perl -slw
use strict;

sub formatYMD {
    my( $y, $m, $d ) = ( localtime shift )[ 5,4,3 ];
    return sprintf "%4d%02d%02d", $y+1900, $m+1, $d;
}

my $today = my $yesterday = time;
$yesterday -= 43200 while substr( localtime( $today ),     0, 3 )
                        eq substr( localtime( $yesterday ), 0, 3 );

my $lastweek = $yesterday - 13 * 43200;
$lastweek -= 43200 while substr( localtime( $yesterday ), 0, 3 )
                       ne substr( localtime( $lastweek ),  0, 3 );

print '    today: ', formatYMD( $today );
print 'yesterday: ', formatYMD( $yesterday );
print ' lastweek: ', formatYMD( $lastweek );

__END__
c:\test>junk4
    today: 20110708
yesterday: 20110707
 lastweek: 20110630
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