To elaborate more ...

> No warnings, no syntax errors, nothing except the (seemingly) impossible behavior

Like already explained, because of the colon else: becomes a label, so it's quite possible.

BUT it's good practice - or at least my practice - to only use uppercase WORDS for labels.

So a linter could easily catch this already.

Furthermore it's pretty hard to use a keyword like else as a label b/c next else; and alike would be syntax errors.

To make it work you'd need to hide it inside an expression like with goto ("else")[0];

So yes, there are potential ways to catch unwanted labels and warn about this. (lowercase and or reserved keyword)

POC: this runs fine

use strict;
use warnings;

my $x;

else:
{
    print ++$x;
    redo ("else")[0] if $x <3;
}
[download]

update

in hindsight I realized that putting the label into quotes is already enough

redo "else" if $x <3;