Some short toying around results in these two short examples that yield no error/warning on ActiveState Perl 5.6.1 (Win32):

1>perl -we "sub rmap(&@){1}; eval { rmap { $_++ } 1 }; print $@;"

2>perl -we "sub rmap(&@){1};        rmap { $_++ } 1"
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Replacing rmap with map yields the error you get in both cases:

3>perl -we "eval { map { $_++ } (1) }; print $@;"
Modification of a read-only value attempted at -e line 1.

4>perl -we "       map { $_++ } (1)             "
Modification of a read-only value attempted at -e line 1.
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But I don't see why my cases 1 and 2 do not raise the error you want.

Update: Of course, ysth is correct, but with my modified variant (5 and 6, replacing 1 and 2), I get the error in both cases, as expected

5>perl -we "sub rmap(&@){my $c=shift;$c->($_) for @_};        rmap { $
+_++ } 1
   print $@;"
syntax error at -e line 1, near "1    print"
Execution of -e aborted due to compilation errors.

6>perl -we "sub rmap(&@){my $c=shift;$c->($_) for @_};        rmap { $
+_++ } 1;
   print $@;"
Modification of a read-only value attempted at -e line 1.
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So maybe your special rmap doesn't call the callback in all cases?