japhy already found the solution..

The perlop manpage tells you a little more about the /e option:

A /e will cause the replacement portion to be treated as a full-fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time. A second e modifier will cause the replacement portion to be evaled before being run as a Perl expression.

MP

In reply to Re: Back substitution expansion by Monky Python
in thread Back substitution expansion by Anonymous Monk

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