it is easy to do with two regexes, such as: ...

Easy, perhaps, but wrong. The suggested two regexes would match a string starting with an underscore.

  DB<1> $f = "_foo";

  DB<2> print "Success" if $f =~ /^[\w\.]+$/ && $f =~ /^\w/;
Success
  DB<3>
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Update (at 7:50 UTC): sorry, the last sentence below is wrong, I had missed the $ at the end of the first pattern.

It would also accept garbage characters in the middle.