Something like this should do it. See the docs and/or ask for anything you do not understand.

#! perl -slw
use strict;

my @grid;
open my $fh, '<:raw', 'the file' or die $!;

while( 1 ) {
    my( $recSize, $dummy, $record );
    sysread( $fh, $recSize, 4 )
        or last;
    $recSize = unpack 'N', $recSize; ##(*)
    sysread( $fh, $record, $recSize ) == $recSize
        or die "truncated record";
    sysread( $fh, $dummy, 4 ) == 4 
        and unpack( 'N', $dummy ) == $recSize  ##(*)
        or die "missing or invalid trailer";

    ## (*) You may need V depending upon which platform your file was 
+created on
    push @grid, [ unpack 'N*', $record ]; 
 }

close $fh;

## @grid should now contain your data
## Addressable in the usual $grid[ X ][ Y ] manner.
## Though it might be $array[ Y ][ X ]
## I forget which order FORTRAN writes arrays in?
[download]