Interval output format

How can I tell Postgres to return an interval value in an format like hhh:mm?

I want to write an application to sum up hours.
I can get the sum with something like

select sum(hours) from table

But the output is '2 10:15'. This is correct, meaning I have worked 58
hours and 15 minutes.

The user needs something like '58:15', and my front end could convert that
(Delphi-Application).
But then I want the reports to be done on the server, using Perl. Good,
Perl can calculate
that. But I have to re implement the conversion, which ist error-prone.

Also it would be nice to have Postgres do the conversion, so that front
ends like psql could
be used.

Thanks for Your help,

Stefan