Nobody has mentioned the Perl Cookbook yet. Recipe 2.1 discusses this topic, I quote here the regex for a C float (does not take IEEE 'NaN' and 'Infinity' values into account) and another suggested way using a wrapper around the POSIX function strtod:

warn "not a C float"
    unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

sub getnum {
    use POSIX qw(strtod);
    my $str = shift;
    $str =~ s/^\s+//;
    $str =~ s/\s+$//;
    $! = 0;
    my($num, $unparsed) = strtod($str);
    if (($str eq '') || ($unparsed != 0) || $!) {
        return;
    } else {
        return $num;
    } 
}
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-- Hofmator