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

use List::Util qw( any );

if (any { $a eq $_ } qw( ABA SCO ACC PHC GHF )) {
    ....
    }