Month Conversion Perl script

Specifications by Ian!    

  $ ./monthconv [ month ]    

  - Program reads the month from standard input if it is not supplied
    as an argument on the command line.    

  - Program truncates input to three characters, so "june" and "Junk"
    would both convert to "6".    

  - Program issues an error message if no conversion is possible.    

  - Program issues an error message if no input is given.    

  - If standard input is not a terminal device:
    - no prompt for input is issued    

  - If standard output is not a terminal device:
    - output consists of *only* the numeric month
      (see examples below)    

    $ ./monthconv December
    The month for 'December' is 12    

    $ ./monthconv jan
    The month for 'jan' is 1    

    $ ./monthconv
    Enter Month (Jan-Dec): NOVember
    The month for 'NOVember' is 11    

    $ ./monthconv ocTEMBER
    The month for 'ocTEMBER' is 10    

    $ ./monthconv garbage
    Cannot find month for input 'garbage'    

    $ ./monthconv
    Enter Month (Jan-Dec): 
    Cannot find month for input ''    

    $ echo July | ./monthconv
    The month for 'July' is 7    

    $ ./monthconv june | cat
    6    

    $ ./monthconv </dev/null
    No input; goodbye.    

    $ echo `./monthconv septober` is the month
    9 is the month    

Use pseudocode and stepwise refinement to sketch how this program should
operate before trying to write real Perl code for it.  You must be
clear on what the program does before you write it.    

    get input
    process input
    print output    

Start from a small, working program and gradually add more features, one
at a time, testing each one as you add it.  DO NOT WRITE THE WHOLE THING
AT ONCE.  You will never get it working.    

    #!/usr/local/bin/perl -w
    $input = <STDIN>;
    print "DEBUG input is '$input'\n";    

Insert debugging print statements to verify that your input is what you
think it is, and that your program is doing what you expect with the data.    

    print "DEBUG input is '$input'\n";
    $input =~ s/foo/bar/;
    print "DEBUG input is now '$input'\n";    

    if ( -t STDIN ) {
        print "Prints only if standard input is a terminal";
    }    

    if ( -t STDOUT ) {
        print "Prints only if standard output is a terminal";
    }    

    $arg = shift;
    if ( defined($arg) ) {
        print "First argument on command line is '$arg'\n";
    }    

    $input = <STDIN>;
    if ( ! defined($input) ) {
        die "No input found; bye-bye.\n";
    }