Perhaps you could show me the steps you would take to tackling the sort of problem I have been trying to get to grips with?

For the purpose of the exercise, I decided to index the words in a bunch of files. To get a start on the exercise, in Perl, I decided to just count the words, here simply defined as whitespace delimited groups of characters, and write the results to a file. This allows me to get something going and I can then work on refining the definition of a "word" and and decide how I am going to store the index.

My first attempt at writing this program (from scratch, just now) was:

#! perl -slw
use strict;
use G; ## Expands wildcards arguments as would be done by most *nix sh
+ells.

our $INDEX ||= 'words.index';

my %index;
if( -e $INDEX ) {
    open INDEX, '<', $INDEX or die "$INDEX: $!";
    m[^([^:]+):(\d+)$] and $index{ $1 } = $2 while <INDEX>;
    close INDEX;
}

while( my $file = @ARGV ) {
    open FILE, '<', $file or warn "$file:$!" and next;
    while( <FILE> ) {
        $index{ $_ }++ for split ' ';
    }
    close $file;
}

my( $key, $val );
open INDEX, '>', $INDEX or die "$INDEX : $!";
print INDEX "$key:$val" while ( $key, $val ) = each %index;
close INDEX;
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I then ran a quick sanity check:

P:\test>perl -c windex-1.pl
windex-1.pl syntax OK
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So far, so good. Now try it out:

P:\test>windex-1.pl *.pl
710:No such file or directory at P:\test\windex-1.pl line 15.
710:No such file or directory at P:\test\windex-1.pl line 15.
710:No such file or directory at P:\test\windex-1.pl line 15.
710:No such file or directory at P:\test\windex-1.pl line 15.
...
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Whoops! A quick look and I see I missed out the keyword pop in line 14:

while( my $file = pop @ARGV ) {
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Try again:

P:\test>windex-1.pl *.pl
306836-trietest.pl:Permission denied at P:\test\windex-1.pl line 15, <
+FILE> line 83342.
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Okay, I've got 1 file with bad permissions in the directory. And what ended up in words.index?

P:\test>type words.index
expletive:2
$midVal:30
9f:8
proven:2
AT:10
happpen:2
inconsequential:2
perverted:2
$x;:24
TTGGGTCAGCGAATGTCACATTTGGAATGGGAATCGGATGATGGGGGCGA:2
++$x):2
greif:2
yvvvvvvvvvvhvevwvvy:2
@exclude_keys:2
$hh2,:2
scrapped:2
classe:2
sod:2
$array[2]->set($_);:2
Cleaner,:2
$expand_ratio,:8
CACGTCTCCACCCGAAGACTGTGGAATGCTACTATTAAAATACTATTTTT:2
cdpr:2
BLANK_NODE:4
paws:6
"\n$_:2
...
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Okay. The word definition is ecclectic, but is seems to be doing what I set out to do, now I can start to refine it. Not a bad start for 10 minutes coding.