Use strict. $temp_page is different from %temp_page.

You can use uniq from List::MoreUtils. Or, make your own:

sub uniq {
    my %tmp;
    undef @tmp{@_};
    return keys %tmp;
}
my (@Category_Link)
    = uniq($main_page =~ m!<a href="http://www.testsite.co.uk/itm/(.+?
+)"!sg);
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Update: Usage line added.