sub modify {
   chomp( $_[0] );
   # whatever else happens, you've just potentially modified $_[0]
}

modify('fred');
# What would you expect to happen to 'fred'?
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Great example. For those who did not try it out, it causes a runtime exception:

Modification of a read-only value attempted at /tmp/t.pl line 2.
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Can modify somehow be declared to only take a modifiable parameter, so that we can have a compile-time error (as happens with the core chomp)?

Can't modify constant item in modify at -e line 1, at end of line
Execution of -e aborted due to compilation errors.
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