Note that if precision is important you might want to peek at the code and run some tests. For the example above, a round trip conversion from DDMMSS => decimal degrees => DDMMSS results in a one second difference.

The problem is with your use of sprintf, or rather %d.

use strict;
use warnings;

use Geo::Coordinates::DecimalDegrees;

my ( $degrees, $minutes, $seconds ) = ( 23, 34, 6 );

my $decimal_degrees = dms2decimal( $degrees, $minutes, $seconds );

printf "%d:%02d:%02.f => %.5f\n", 
    $degrees, $minutes, $seconds, $decimal_degrees;

( $degrees, $minutes, $seconds ) = decimal2dms( $decimal_degrees );

printf "%.5f => %d:%02d:%02.f\n", 
    $decimal_degrees, $degrees, $minutes, $seconds;

__END__
c:\test>junk5
23:34:06 => 23.56833
23.56833 => 23:34:06
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The module doesn't int the seconds, allowing for input and output of fractional seconds. If you don't want the fractions, you should use %.f rather than %d. And while your at it, it's conventional to include leading zeros on coordinates, hence %d:%02d:%02.f.