foreach my $filename (qw{gravity magnetics bathymetry}) {
    open my $file, '<', $filename or die $!;
    while (<$file>) {
        chomp;
        my @a=m{ \A ( \S+ \s \S+ \s \S+ \s \S+ ) \s ( \S+ ) \z }msx;
        $merged{$a[0]}//=[ qw{null} x 3 ];
        $merged{$a[0}}[$index]=$a[1];
    }
    close $file;
    ++$index;
}