But this works

my @letters = qw(alfa beta gama);

for my $letter (@letters){
   warn "iteration is ", scalar($0 .. !$0);
}

for my $letter (@letters){
   warn "iteration is ", scalar($0 .. !$0);
}
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I think the range-operator depends on the code position, but your sub is always at the same one. *

UPDATE: well the documentation could be clearer...
perlop

... The sequence number is reset for each range encountered ....

...in the compilation phase!!!

(*) that's why the scalar range is known as flip-flop-operator