Here's the code that I used when I needed to do this:

if($mday =~ /(?<!1)1$/)    { $date_word = $mday . 'st' }
elsif($mday =~ /(?<!1)2$/) { $date_word = $mday . 'nd' }
elsif($mday =~ /(?<!1)3$/) { $date_word = $mday . 'rd' }
else                       { $date_word = $mday . 'th' }
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Using a module (as mentioned above) is probably a better idea, however...