eval { 
  require DBI;
  DBI->import;
  # ... as is documented in perlfunc under 'use',
  # this is same as 'use DBI', except done when the eval {}
  # is run, not when compiled (eval BLOCK catches only
  # runtime errors, eval STRING must be used to catch 
  # errors that  happen at compile time, but eval BLOCK 
  # is almost always better.)
};
if ($@) { # $@ has error from eval, if any.
  # Couldn't load DBI
} else {
  my $dbh = DBI->connect(...); 
  # Loaded DBI, okay.
}