use Regexp::Common;

my %units = map ($_,1), qw[ miles kilometers pounds kilograms ... ];

my( $num, $from, $to ) = getUserInput();

die 'Bad number input' unless $num =~ /^\s*$RE{num}{real}\s*$];
die 'Bad units input'  unless exists $units{ lc $from } 
                        and    exists $units{ lc $to );

my $res = `units $num $from $to`;