The behaviour of

my $x = 0 if 1 == 2;
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is documented as undefined.

The problem is that you get the compile-time effect of my (declaring the variable) without the run-time effect (clearing the variable).

Lexicals variables are permanently allocated. They are cleared on scope exit, not freed.

The recursive effects are related to existence of a copy of the lexicals for every layer of recursion.

sub f {
  my ($n) = @_;
  my $x if 0;
  print ++$x, "\n";
  f($n-1) if $n>1;
}

f(3); # 1,1,1
f(2); # 2,2
f(3); # 3,3,2
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Update: Added bit about recursion.