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$_ = lc for @x;

@x = map lc, @x;

lc($_);                # No effect. void warning
$_ = lc($_);           # Modifies $_. no warning
s/.*/lc($_)/e;         # Modifies $_. no warning
s/.*/lc($_)/er;        # No effect. void warning
$_ = s/.*/lc($_)/er;   # Modifies $_. no warning