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

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

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

    ++$index;
}

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