Tip #6 from the Basic debugging checklist: Check to see if your code is what you thought it was

perl -MO=Deparse -p code.pl

sub aa {
    2;
}
sub bb {
    2;
}
LINE: while (defined($_ = <ARGV>)) {
    my($a, $b) = (aa(), bb());
    my($s, $t) = (2 * aa(bb()), 2 * $a + $b);   # <-------- bb is inpu
+t to aa
    'STDERR'->say("$a==", aa());
    'STDERR'->say("$b==", bb());
    'STDERR'->say("$s==$t");
}
continue {
    die "-p destination: $!\n" unless print $_;
}
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One fix: add parens:

#               v    v
my ($s, $t) = (((2*aa)+bb), (2*$a+$b));
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