I think it's more likely that there is another part of your code that interferes with the code that you showed us.

If you want us to test that you'd have to provide a sample script that we can execute and that demonstrates your problem.

there really isnt much else to my code:

#!/usr/bin/perl -w
use strict;
use BerkeleyDB;
use WWW::Mechanize;
my $mech = WWW::Mechanize->new();

my %pageHash;

tie %pageHash,  "BerkeleyDB::Btree",
                -Filename => 'pageContents',
                -Flags    => DB_CREATE,

        or die "Cannot open file pageContents: $! $BerkeleyDB::Error\n
+" ;

my %hash;


tie %hash,  "BerkeleyDB::Btree",
                -Filename => 'videoDB',
                -Flags    => DB_CREATE,

        or die "Cannot open file videoDB: $! $BerkeleyDB::Error\n" ;

foreach my $x (keys %hash)
  {
    print "getting $x:";
    my $data = $hash{$x};

    my @parts = split(/\t/, $data);

    my $url = $parts[0];
    $mech->get( "$url" );
    if($mech->success())
      {
        #$pageHash{$x} = $mech->content( format => 'text' );
        print " done\n";
      }
    else
      {
        print " failed\n";
      }
    sleep(15);
  }
[download]

I tested your code (I added two URLs to the hash) and I can't reproduce your problem. Neither key nor value get modified in any strange ways.

Maybe try to move your pageContents files and start with an empty, clean one and check if the error still occurs.

I switched to LWP::Simple, and that seems to work no problem. I didnt change anything else, so this behavior remains mysterious.