Using the core module Module::Load:

use Module::Load;

my $state = 'TX';
my $module = 'State::' . $state . '::formulas';
eval {
    load $module;
} or do {
    my $err = $@;
    ...
};
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There are more ways to do it, this is one :-) (update: huck showed an even lower-level one while I typed, ++)