OK, I think I got it working. forget Tie::Watch, I could not get that to work (though I am sure it can be made to) but I have created a little package called Sensitive_Array. this will set a global variable when the watched array is altered, dirty I know but that's the way the code grew. It also prints out the methods being called. I did not implement it all, but more than enough for this example.

Make @watched_array the array your code is using, do not touch @array directly

First the test code

#!/usr/bin/perl
use strict;
use warnings;
use Tie::File;
use Sensitive_Array;

# this is the global var I am using
$main::touched=0;
my (@array, @watched_array);
my $file='test_tie_file';

# First tie the array to the file
tie @array, 'Tie::File', $file or die "cannot tie to $file: $!\n";
push @array, int (rand 1000) until $#array > 39; # fill it on the firs
+t pass

# now tie it up to an array that will tell us when it was altered
tie @watched_array, 'Sensitive_Array', \@array;

print "array of length $#watched_array is now watched\n";

print "would you like to alter the file (y/n)\n";
my $ansa=<STDIN>;
push @watched_array, 'touched' if $ansa =~/^y/;

untie @array;

if ($main::touched) {
    print "File has been altered\n"
} else {
    print "File remains the same\n";
}
[download]

C:\PERL\bin>perl tie_test
 Fetchsize array of length 47 is now watched
would you like to alter the file (y/n)
y
 Push  Fetchsize File has been altered

C:\PERL\bin>perl tie_test
 Fetchsize array of length 48 is now watched
would you like to alter the file (y/n)
n
File remains the same
[download]

Cheers,
R.

I guess you can not do the obvious and patch in a flag to your code that is flipped by the same logic that decides whether to alter the tied array ?

use Tie::File;        
tie @array, 'Tie::File', filename or die ...;

if ($something) {
    push @array, $something_else
    $file_altered++;
}
[download]

Another other way would be to use Tie::Watch to notify you if changes are made to the file tied array. I am assuming you can chain these two together, will try some test code myself and report back later if I get time.

you could also do an MD5 hash of your tied array when you first open it and compare this to one before you untie it. This is going to be a lot more costly than comparing the modification times.

Update

#!/usr/bin/perl
use strict;
use warnings;
use Tie::File;
use Tie::Watch;

$main::touched=0;
my @array;
my $file='test_tie_file';
sub touched {$main::touched=1}


push @array, int (rand 1000) until $#array > 39; # fill it on the firs
+t pass

my $watch = Tie::Watch->new(
     -variable => \@array,
     -debug    => 0,
     -shadow   => 1,
     -fetch    => sub {print "Fetch\n";   
                   my $self=shift; 
               $self->Fetch},
     -store    => sub {print "Store\n";
                   $main::touched=1; 
               my $self=shift; 
               $self->Store (@_)},
     -push     => sub {print "Push\n";    
                   $main::touched=1; 
               my $self=shift; 
               $self->Push (@_)},
     -clear    => sub {print "Clear\n";   
                   $main::touched=1; 
               my $self=shift; 
               $self->Clear   (@_)},
     -extend   => sub {print "Extend\n";  
                   $main::touched=1; 
               my $self=shift; 
               $self->Extend  (@_)},
);

tie @array, 'Tie::File', $file or die "cannot tie to $file: $!\n";
$main::touched=0;
print "array of length $#array is now watched\n";

print "would you like to alter the file (y/n)\n";
my $ansa=<STDIN>;
push @array, 'touched' if $ansa =~/^y/;

untie @array;
$watch->Unwatch;

if ($main::touched) {
    print "File has been altered\n"
} else {
    print "File remains the same\n";
}
[download]

Cheers,
R.