$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 )  )