Just move the variable declaration above the first if modifier e.g

my @xxx;
@xxx = (one, two, three) if ( $op eq "numbers" );
@xxx = (four, five) if ( $op eq "alpha" ); 
@xxx = (six, ten) if ( $op eq "alp" );
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You'll no longer get the warning as you won't be declaring the @xxx multiple times (as lexicals are declared at compile-time (i.e when it's putting the program together)).