I think it is clearer and less error-prone to open the file twice (input, then clobber and output) instead of opening once in read-and-write mode.
Working, tested code:
#!perl
use strict;
use warnings;
my $filename = '/tmp/test.index';
my $index = 0;
if ( -e $filename ) {
open my $input_fh, '<', $filename
or die "Failed to open '$filename' for input: $!";
my $line = <$input_fh>;
if ( not eof $input_fh ) {
die "File '$filename' has more than one line! Dying to avoid c
+lobbering unexpected contents.";
}
close $input_fh or warn;
chomp $line;
$index = $line;
}
$index++;
open my $output_fh, '>', $filename
or die "Failed to open (and clobber) '$filename' for output: $!";
print {$output_fh} $index, "\n";
close $output_fh or warn;