in reply to Help choosing the most efficient, dependable condition(al)

Without trying to dumpster-dive into this particular debate, I would note that it is easy to avoid messages like “use of uninitialized value” through the use of the && (logical-AND) operator, which uses so-called short circuit evaluation:   if the left-hand side is False, the right-hand side is not evaluated at all.   (See perldoc perlop.)

Thus, you can say (say...):
if ( $ENV{'HTTP_ACCEPT'} && ( $ENV{'HTTP_ACCEPT'} =~ ...

undef is always False.   So, the statement quits right there and the remainder of the expression is never evaluated.

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Re^2: Help choosing the most efficient, dependable condition(al)
by boftx (Deacon) on Nov 13, 2013 at 21:44 UTC

    The preceding is a good way to go, but it you want slightly simpler I've done this before: 

    if (($ENV{'HTTP_ACCEPT'} || '') =~ /application\/xhtml\+xml/)
{
    print "content-type:application/xhtml+xml; charset=utf-8\n\n";
}else{
    print "content-type:text/html; charset=utf-8\n\n";
}

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