(Blocks which aren't even mutually exclusive, I might add.)

Initially I thought you meant that the blocks aren't mutually exclusive from the perspective of lexical variable scoping. It has never occurred to me that an if statement might create a single scope, rather than one for each branch. Testing seems to confirm my assumption that each branch is a separate scope.

#!/usr/local/bin/perl
use strict;
use warnings;

foreach (1..3) {
    if($_ < 2) {
        my $x = 1;
        print "$_ < 2, \$x = $x\n";
    } else {
        print "$_ >= 2, \$x = $x\n";
    }
}
__END__
Global symbol "$x" requires explicit package name at ./test.pl line 10
+.
Execution of ./test.pl aborted due to compilation errors.
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Then it occurred to me that you probably mean that the conditions of the two if statements are not mutually exclusive - a possible problem with program logic but not affecting variable scope.