Generally speaking, if you're making numerous changes to a string, $_ helps. And you can clarify by choosing a delimiter that isn't in your string, so you don't have to escape it.

foreach (@dates) {
    s|^\s+||;    #remove leading space 
    s|\s+$||;    #remove trailing space
    s|\s|/09 |;  #add the year
    s|-|/|;      #change - to /
    ...
}
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But the other responses are better...