map {
    open my $file, '<', $_ or die $!;

    map {
        $merged{$_->[0]} //= [ qw{null} x 3 ];
        $merged{$_->[0]}[$index] = $_->[1];
    }
    map {
        [ m{ \A ( \S+ \s \S+ \s \S+ \s \S+ ) \s ( \S+ ) \z }msx ]
    }
    map {
        chomp; $_
    } (<$file>);

    close $file;

    ++$index;
} qw{gravity magnetics bathymetry};

##</code><code>##

foreach my $filename (qw{gravity magnetics bathymetry}) {
    open my $file, '<', $filename or die $!;

    map {
        $merged{$_->[0]} //= [ qw{null} x 3 ];
        $merged{$_->[0]}[$index] = $_->[1];
    }
    map {
        [ m{ \A ( \S+ \s \S+ \s \S+ \s \S+ ) \s ( \S+ ) \z }msx ]
    }
    map {
        chomp; $_
    } (<$file>);

    close $file;

    ++$index;
}

##</code><code>##

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;
}