if you are only using single character command line switches then I believe #!/usr/bin/perl -s will enable what the docs call "rudimentary switch parsing". basically (and this worked for me) perl puts a true value into a variable named the same as the switch so...

#!/usr/bin/perl -s
if ($a) { print "user selected a\n"; }
elsif ($b) { print "user selected b\n"; }
elsif ($c) { print "user selected c\n"; }
else { print "no args!\n"; }
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Command line:

[larryk@home] /home/larryk > somescript -a
user selected a
[larryk@home] /home/larryk > somescript -b
user selected b
[larryk@home] /home/larryk > somescript -c
user selected c
[larryk@home] /home/larryk > somescript
no args!
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hope this helps.