You don't need clutter just to use a variable. Remember that a do block returns the last evaluated expression. In this example, I assign that result to @memories, which puts the matches in list context so they return the parts they remembered as a list.

$_ = 'abcdefghijk';

foreach my $left ( 0 .. 1 )
    {
    my @memories = do {
        if( $left ) { /ab(..)ef/ }
        else        { /fg(..)jk/ }
        };
    
    print "Left $left: caught @memories\n"
    }
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Once I have the memories in the array, I can easily pull out any element that I like. If the match fails, @memories is empty. I don't have to deal with a lot of gymnastics to figure out is the match succeeded. Without knowing that the match suceeded, you shouldn't use any of the number variables ($1, $2, and so on).

Remember that $1 and so on are package variables, and that they persist until the next successful pattern match. It doesn't have anything to do with lexical scope. Actually, perlvar says:

$<digits> Contains the subpattern from the corresponding set of capturing parentheses from the last pattern match, not counting patterns matched in nested blocks that have been exited already. (Mnemonic: like \digits.) These variables are all read-only and dynamically scoped to the current BLOCK.