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$dir= $ENV{TMPDIR} || "/tmp";
$file ||= "default";

$size= defined $ENV{MAXSIZE} ? $ENV{MAXSIZE} : -1;
$size= -1   if  ! defined $size;

$size= def( $ENV{MAXSIZE}, -1 );
def( $size, -1 ); # void context causes $size to be modified
...
# 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 )  )

sub def {
    my( $val, $alt, $force )= @_;
...
    $_[0]= $val   if  ! defined wantarray  ||  $force;
    return $val;
}