sub def {
    my $var= \$_[0];
    my $val;
    while(  @_  &&  ! defined $val  ) {
        $val= shift @_;
    }
    $$var= $val
        if  ! defined wantarray;
    return $val;
}

# For example:
my $size= def( $ENV{MAXSIZE}, -s $swapFile, -1 );

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