Very close but you can't do it quite that way. Here is your code in working form:

$source =~ m!(\d{1,2})[-/](\d{1,2})[-/](\d{2,4})!g;
my $day = $1;
if( $day == 1 or $day == 21 or $day == 31 ){
    $day .= 'st';
}elsif( $day == 2 or $day == 22 ){
    $day .= 'nd';
}elsif( $day == 3 or $day == 23 ){
    $day .= 'rd';
}else {
    $day .= 'th';
}
[download]

I have shown you the use or character classes and different regex delimiters (you can use ! or anything else for that matter, not just /) as well as the .= operator. A little indenting makes it easier to read. The last case is just an else as we want to catch everything that is not a st/nd/rd with th

This also works and demonstrates an alternative to the if elseif else structure for simple stuff

$day .= ( $day == 1 or $day == 21 or $day == 31 ) ? 'st' :
        ( $day == 2 or $day == 22 ) ? 'nd' :
        ( $day == 3 or $day == 23 ) ? 'rd' :
        'th';
[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print