my @files = glob "*.txt";
my %file_idx = map {; $_ => index_file( $_, 5 ) } @files;

=pod

{    'foobar.txt' => {
        word => [ 1, 3, 5, 6 ],
        another => [ 5, 7, 2, ] },
    'barfoo.txt' => {
        .......
}

=cut

sub index_file {
    my $filename = shift;
    my $lines_of_context =
        $_[0] > 0
        ? shift()
        : 1;
    open my $fh, "<", $filename or die "Couldn't open $filename: $!";

    my @offsets;
    my %index;
    while ( my $line = <$fh> ) {
        push @offsets, tell $fh;
        my $offset =
            scalar( @offsets ) < $lines_of_context
            ? $offsets[0]
            : shift @offsets;
        for my $word ( split ' ', $line ) {
            push @{ $index{$word} }, $offset;
        }
    }

    close $fh or warn "Couldn't close $filename: $!";
    return \ %index;
}