I assume that what you call the "actual string" is a description of the pattern you would like to match. What you have is not bad, but you could do the following (used the "x" option to add spaces and comments for readability):

if ($ARGV[0] =~ /^ep_svr_
                 (\d)      # any single-digit number -- use  (\d+) for
+ multi-digits
                 _([A-Za-z]{3})  # three letters
                 _([A-Za-z]{3})  # three letters
                 _(\w)     # any "word" character -- use (.) for any c
+haracter
                 \z         # if you want to anchor the end
               /x) {
   print "matched ",$ARGV[0],"\n";
   my ($num,$alpha1,$alpha2,$char) = ($1,$2,$3,$4);
   # Do something with the captured parts here...
}
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Update: I made use of your capturing parentheses -- otherwise there is no need to use them.