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while ( <DATA> ) {
    chomp( $_ );             #get rid of whitespace at end
...
    #ThreeDHash[ GROUP ][ PROJ  ][ MONTH ] = DATA
    $ThreeDHash{ $f[0] }{ $f[1] }{ $f[2] } = $f[ 4 ];
}

#same as a hash, only with square brackets 
# dont forget $index1/2/3 all have to be integers
# so you will have to index every entry to the file.
$ThreeDArray[ $index1 ][ $index2 ][ $index3 ] = $DATA;