Warnings can be very helpful; I like them. But dealing with undefined values without triggering a warning can be a slight pain. While the following:

$dir= $ENV{TMPDIR} || "/tmp";
$file ||= "default";
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work in many cases, they don't work when you need to distinguish certain false values from undefined. Now the following:

$size= defined $ENV{MAXSIZE} ? $ENV{MAXSIZE} : -1;
$size= -1   if  ! defined $size;
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aren't so bad but require repeating either the variable name or the value. So def() gives you something that works very much like || and ||= but based on definedness, not falseness:

$size= def( $ENV{MAXSIZE}, -1 );
def( $size, -1 ); # void context causes $size to be modified
if(  0 < def( $size= $ENV{MAXSIZE), -1, 1 )  )
# The third argument above forces $size to be modified
# even though no void context used.  An alternative is:
if(  0 < ( $size= def $ENV{MAXSIZE), -1 )  )
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sub def {
    my( $val, $alt, $force )= @_;
    $val= defined $alt ? $alt : ""   if  ! defined $val;
    $_[0]= $val   if  ! defined wantarray  ||  $force;
    return $val;
}
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