How to troubleshoot authentication failure?

I have two Linux servers that are pretty similar to each other, and both are
running PostgreSQL servers, but in one server a certain Perl script succeeds
in connecting to the localhost server whereas in the other one the same
script fails. The error on the second server is of the form "fe_sendauth:
no password supplied".
I have verified all the possible reasons that I can think of for why this
script succeeds on one server but fails on the other. In all these respects
the two set ups are absolutely identical: same pg_hba.conf files, same
script parameters, same versions of Postgres, same everything.

I have run out of ideas!

How can I troubleshoot this further? Is there some way to have Postgres
dump excruciatingly thorough details about every single step of the
authentication sequence?

Any other suggestions would be greatly appreciated.

Thanks!

k