use warnings;
use strict;
use Number::FormatEng qw(format_eng);

for my $num qw(1.23e2 1.234 1234.56 1234567 6.66e+29) {
    printf "%-10s -> %-10s\n", $num, format_eng($num);
}

__END__

1.23e2     -> 123
1.234      -> 1.234
1234.56    -> 1.23456e3
1234567    -> 1.234567e6
6.66e+29   -> 666e27