use strict;
use warnings;

my @words = qw(book reboot rocket);
my @chars = map {[split '']} @words;
my %matches;

for my $row (0 .. $#chars) {
    for my $column (0 .. $#{$chars[$row]}) {
        push @{$matches{$chars[$row][$column]}}, [$row, $column];
    }
}

for my $char (sort keys %matches) {
    next unless @{$matches{$char}} > 1;
    
    print "$char=", join (',', @$_), "\n" for @{$matches{$char}};
}