Scan the data to find the coordinates for each character and save the results in a hash of arrays (one hash entry for each character encountered). Then run through the hash and generate the report:

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}};
}
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Prints:

b=0,0
b=1,2
e=1,1
e=2,4
k=0,3
k=2,3
o=0,1
o=0,2
o=1,3
o=1,4
o=2,1
r=1,0
r=2,0
t=1,5
t=2,5
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