#!usr/bin/perl    -w-T

#ABOUT
# Objective: renames all .csv files (from MS Excel) in a given
#directory to <StationNumber><Parameter>.csv where <StationNumber> and
#<Parameter> are taken from the file contents.

#DETAIL
# Abstracts relevant information from a file, re-names the file to
#(typically):
#<GaugeRefNo>REev.csv for tipping-bucket rainfall gauges.
#<GaugeRefNo>RSst.csv for storage rainfall gauges.

#CODE
use strict;
use diagnostics;
use Cwd;

my %renamefiles=();
my $renamefiles;
my @keys=();
my $key;

{
my $files;
my @files=<c:/Storage/*.csv>;    #Reads all csv files

foreach(@files)
    {    my $open=$_;    #Read current file to process from array
        my $string;    #Slurp file line by line
        my $gaugeref;
        my $gaugeparam;
        my $currentdir;
        my $newdir;
        my $chaff;
        my $result;
        my $newname;
        my $oldname;
######TODO - Read $param from file contents
#        my $param="REev";    #For tipping bucket raingauges
        my $param="RSst";    #For storage raingauges
#        my $param="SG";        #For stage (level)
######
        my $extension="csv";

        #Strip filename to "/" in path
        $newdir=$_;$chaff=chop($newdir) while substr($newdir,length($n
+ewdir)-1)!~/\//;
        chdir $newdir;
                
        #PARSE IN
        open(INFILE,"<", $open) or die "Couldn't open $open $!";
        print "Opened $open \n";
        while(<INFILE>)    #Loop through lines
            {    $string=$_;
                # GET STATION NUMBER
                if (lc($string)=~ /(gauge ref|station number)/)
                    {    my $station=$string;    
                        chomp $station;
                        $chaff=chop($station) while substr($station,le
+ngth($station)-1)=~/\D/;    #drop chaff
                        $result.=chop($station) while substr($station,
+length($station)-1)=~/\d/;    #append hits
                        $gaugeref=reverse($result);
                        next;
                    };
            };
        close $open;
        $result="";#Clear
        $result.=chop($open) while substr($open,length($open)-1)!~/\//
+;#read filename from path until encounters /
        $oldname=reverse($result);
        $newname="$gaugeref$param.$extension";
        print "Old:$oldname\nNew: $newname\n";
        %renamefiles=($oldname=>$newname);

        @keys=sort keys %renamefiles;
        foreach $key(sort keys %renamefiles)
            {
            if (!(-w $oldname))
                {print "$oldname is not writeable\n";}
            else{
                use Stat::lsMode;
                my $mode = file_mode($oldname);
                print "Filemode is $mode\n";
                renamefile("$key","$renamefiles{$key}");
                };
            };
        };
};
    
######SUBS######
    sub renamefile{
        my $oldname=shift;
        my $newname=shift;
        my $success="";
        
        print qq{Attempting to rename $oldname to $newname\n};
        $success=rename $oldname, $newname;
        if ($success){print "Changed $oldname to $newname\n"}
        else
        {print "Failed to rename $oldname to $newname $!\n";exit};
    };
[download]

This isn't quite what I had in mind when I said small example, but I guess it'll do. One thing I see right away is that you're not closing the file after you open it. As previously mentioned, Windows doesn't like to rename open files. Where you have:

        close $open;
[download]

You should have:

        close INFILE;
[download]

The "close" does it...(goes PURPLE with embarrassment and goes off to say ten "Hail Randall"s and three "Our Larry"s)

I'm amazed at the interest in this problem and everyone's willingness to help me out. Note to self to contribute wisdom if ever possible.

Thank you very much indeed...