Perl::Tidy to the rescue.

Here's the code in a slightly more readable format. Will let other monks carry the torch from here:

#!/usr/bin/perl
# Syntax : /../perl /.../PS_file_Rename.pl -s /..../Source_directory -
+f filename.xls
# .. -> path of Perl.exe
# ... -> path of the script PS_file_Rename.pl
# -s option - Source_Folder
# -f option - Filename to use for renaming

use Getopt::Std;
use Data::Dumper;
use POSIX qw(strftime);

use vars qw($opt_s $opt_f);

getopts('s:f:');
$ff = "$opt_s" . "\\" . "$opt_f";

#if the file name already exists delete it
if ( -e $ff ) {
    unlink($ff)
      or warn scalar localtime( time() ), " WARN Can't delete $ff : $!
+";
}
my $path = $opt_s . "\\*.*";
$path =~ s|\\|/|g;

#find the newest file
my $newestfile = (sort {(stat $b)[10] <=> (stat $a)[10]} glob $path)[0
+];

#rename the newest file to the name passed
rename( $newestfile, $ff )
  or warn scalar localtime( time() ), " WARN Can't rename $newestfile 
+: $!";
@files = glob $opt_s . "\\*.*";
foreach my $file (@files) {
    if ( $file ne $ff ) {
        unlink($file)
          or warn scalar localtime( time() ),
          " WARN Can't delete " . $opt_s . "\\" . $file . " : $!";
    }
}
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