map does return a list, but it returns the value of the expression or block. In your case this would be the result of the s///, which would be a true or false value (1 or undef) depending if the substitution succeeded or failed. Also keep in mind that $_ is an alias for each element of the list being map'd over, similar to how $_ behaves inside of a foreach. This means that your substitution is modifying your original list.

@a = qw(a b);
@b = map { s/([ac])/uc $1/e } @a;
print join( ",", map {length($_) ? $_ : q{""}} @{$_} ), "\n"
  foreach \@a, \@b;
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As for using map in void context (throwing away its return value) is considered bad form in many circles. Either use foreach to iterate over the list (using push to build a new list as you go), or use a temporary copy to do the subistitution on inside the map.

@mod_lines = map { (my $tmp = $_) =~ s/^#.*//; $tmp } @all_lines;
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