print "Enter the output file you would like to analyze: ";
chomp (my $filename = <STDIN>);
print "Enter the isotope you want to extract (ex: Am-241): ";
chomp (my $iso= <STDIN>);

##</code><code>##

# Validate parameters
@ARGV == 2 or error ("Too few parameters");
$ARGV[0] =~ /[A-Z][a-z]?-\d{1,3}/ or error ("Expected an isotope first");
-f $ARGV[1] or error ("<$ARGV[1]> is not a file");

# Extract parameters
my ($iso, $filename) = @ARGV;

...

sub error {
    my $msg = shift;

    print <<"USAGE";
Error: $msg.

ExtractIso parses a given dibbly report file and extracts the
record for a given isotope. Use as:

    ExtractIso <iso> <filename>

<iso> is the isotope to be extracted. For example Am-214.

<filename> is the filename (with path as required) of the
record file.

For example:

    Extract Am-214 dibbly.dat

would extract and print the record for Am-214 from the 
dibbly.dat file in the current directory.
USAGE

    exit -1;
}