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# Suppose $op is the name of the sub,
# and $arg is the argument string to pass to it.
if (is_a_safe_sub($op))
...
    no strict 'refs';
    $op->($arg) or warn "Error, blah, blah";
}

if (is_a_safe_sub($op))
{
    eval "$op($arg)" or warn "Error, blah, blah";
}