That would be a good solution for any reasonably current version of Perl. I'm not exactly sure when the POSIX [:blah:] notations were incorporated into perl's regex engine, but I know some people are running perl environments where it is not available:

$ perl -v

This is perl, version 5.005_03 built for sun4-solaris

Copyright 1987-1999, Larry Wall

...

$ perl -le '$_="abc"; print "ok" if (/^[[:ascii:]]+$/)'

# (no output -- that char.class doesn't work)

$ perl -le '$_="abc"; print "ok" if (/^[\x00-\x7f]+$/)'
ok
[download]

The OP didn't actually say how far back he needs to go with his perl version(s), but I'm not surprised if 5.005 is part of his condition.

(Update: forgot to mention: that same sun4-solaris machine also has perl 5.8.0 installed as a non-default version, and the first one-liner works with that as expected:

$ perl5.8.0 -le '$_="abc"; print "ok" if (/^[[:ascii:]]+$/)'
ok
[download]