my %expr = (
    EX_CASE1 => { map { $_ => 1 } qw( ABA SCO ACC PHC GHF ) },
    EX_CASE2 => {}, # …
    );
:
:
if (exists $expr{EX_CASE1}{$a}) {
    ...
    }
:
if (exists $expr{EX_CASE2}{$b}) {
    ...
    }