In your first example, you grep all of your "large" files, and then use only the first result. How many large files are you opening unnecessarily?

How does this compare:

   my $hba_alias = `for f in $switchdir/*.swc ; do grep $host_hi \$f &
+& break; done |grep ":   device-alias"`;
[download]

That's analogous to graff's last if $hba_alias;