I get different results depending on whether or not $f is quoted in the for list:

sub f2a {
my ($f) = @_;
for my $d ("$f") {return $d}
}

sub f2b {
my ($f) = @_;
for my $d ($f) {return $d}
}

print map {f2a($_)} qw(x y z); # zzz
print map {f2b($_)} qw(x y z); # xyz
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I also ran it through B::Deparse, but gained no insight.