You could use the /g regex modifier to overcome the 'only periods' assumption....

sub get_nsentences {
  my $text = shift;
  my $count = shift || 1;
  join '', ($text =~ /.*?[\.!?]/sg)[0..$count-1]
}
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This keeps the punctuation with the "sentence" so you don't have to keep track of what it was.... i.e. you can join with '', not with '.'

-Blake