use strict;
use warnings;
use Scalar::Util qw( looks_like_number );

# .....

if( $temp =~ m/^([0-9.+-]+)\s*([cf])/i and looks_like_number( $1 ) ) {
    my( $number, $system ) = ( $1, lc $2 );
    # Do whatever you want here; 
    # $number contains the temp, 
    # and $system contains 'c' or 'f'.
}
else {
    die "Invalid number format.";
}