# populate filename array
my @filenames = qw(file1 file2 file3 file4 file5 file6);

my %next;
my %prev;

# Link next to first filename
$next{$filenames[0]} = $filenames[1];

# Link next and prev to all the filenames
for (my $fileindex = 1; $fileindex < scalar (@filenames) - 1; $fileindex++) {
  $next{$filenames[$fileindex]} = $filenames[$fileindex + 1];
  $prev{$filenames[$fileindex]} = $filenames[$fileindex - 1];
}

# Link prev to last filename
$prev{$filenames[-1]} = $filenames[-2];