or: my $page = $query->param("page") || '';

Caveat: That will also assign '' to $page if $query->param("page") returns 0 or "0" (the same applies to hippo's answer).

TIMTOWTDI: Another alternative to the code from the OP may be (at least on Perl v5.12 and up):

sub loadPage {
    my $page = $query->param("page");    
    if ( length($page) && $page eq 'main' ) {
        loadMainPage();
        return 1;
    }
}
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