Comments are inline.

use strict;
use warnings;
use Storable;
use vars qw($CONTEXT_LINES $TEXTS %STOPWORDS $INDEX);

$CONTEXT_LINES = 3;
@STOPWORDS{
    # Prefer reading these from a file.
    qw( the a an of and on in by with at he after into
        their is that they for to it them which) }
    = ();


# prefer texts/*.txt over "chdir 'text'; glob '*.txt'"
my @files = glob "texts/*.txt";
my %file_idx = map {; $_ => index_file( $_, $CONTEXT_LINES ) } @files;

store \%file_idx, "../index/text.idx";

=pod

{    'text.idx' => {
        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;

        # Prefer ' ' over /\s+/ here. See perlfunc about this.
        for my $word ( split ' ', $line ) {
        $word = lc $word;
            # Prefer character classes to alternation when possible
        $word =~ s/[,.]$|[\][();:!]//g;

            next if exists $STOPWORDS{$word}
                 # Prefer (\d+) to (\d)+ (unless that is *really* what
+ you mean)
                 or $word =~ /p\.(\d+)/
                 # Remove the '?' as that makes the operation always s
+ucceed.
                 or $word =~ /-{5,}/;

            push @{ $index{$word} }, $offset;
        }
    }

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