#!/usr/bin/perl
use strict; use warnings;

my $depth  = 0;
my $indent = ' ' x 4; # four spaces

LINE:
while ( <> ) {
    next unless /^=/;

    if ( /^=over/ ) {
        $depth++;
        next LINE;
    }
    elsif ( /^=back/ ) {
        $depth--;
        next LINE;
    }

    s/=item //;

    print $indent x $depth . $_;
}