An alternative to Date::Manip is the Date::Calc module which also provides hordes of date utility functions (and it is a C lib so it is fairly quick).

#!/usr/bin/perl -w
use strict;
use Date::Calc qw/Day_of_Week/;

my @days = qw/Sunday Monday Tuesday Wednesday
              Thursday Friday Saturday/;

my $dow = $days[ Day_of_Week(2002, 5, 25) ];
print "$dow\n";
__END__
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Several have mentioned simple ways to do it yourself, but if you need more date manipulation than your post suggests (things like differences between dates, business days, etc), I recommend checking out one of the modules -- it isn't hard to work with dates, but it isn't hard to work with dates incorrectly either :-)