I never heard about an automatic resetting of the alarm interval. I know about a different automatic behavior: Perl will automatically resume some system calls after receiving an ALRM signal; that's why the SIG{ ALRM } handler has to explicitly die. For example, this one-liner hangs:

% perl -le '$|=1; $SIG{ALRM} = sub { print q(Goosed!) }; alarm 2; eval
+ { <> }; print q(done)'
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because after the ALRM perl goes back to waiting for input. The one-liner also illustrates that the alarm interval is not automatically reset after the program receives an ALRM signal, since the $SIG{ ALRM } handler is called only once.