You are quite correct. The faulty solution can only happen with n >= 10; it happens because 'e10' isn't attacked by 'f1', and "e10" =~ /e1/. Luckely, the fix is simple:
```#!/usr/bin/perl

use strict;
use warnings 'all';

use Getopt::Long;

Getopt::Long::Configure ("bundling");
GetOptions ('p|print'    =>  \my \$print,
'P|Print'    =>  \my \$Print,
'n|number=i' => \(my \$nr_of_queens = 8)
);

my @rows =               1 .. \$nr_of_queens;
my @cols = ('a' .. 'z') [0 .. \$nr_of_queens - 1];

sub a2i {ord (\$_ [0]) - ord ('a') + 1}
sub i2a {chr (\$_ [0] + ord ('a') - 1)}

# Given a square, return all non-attacked squares on columns to
# the *left* of the given square. (a1 is the lower left corner).
sub free {
my (\$C, \$R) = \$_ [0] =~ /(\D)(\d+)/;
\$C = a2i \$C;
map {join "" => i2a (\$_ -> [0]), \$_ -> [1]}
grep {\$_ -> [0] != \$C &&
\$_ -> [1] != \$R &&
abs (\$_ -> [0] - \$C) != abs (\$_ -> [1] - \$R)}
map {my \$c = a2i \$_; map {[\$c, \$_]} @rows} @cols [0 .. \$C - 1]
}

my (\$str, \$re) = ("", "");

foreach my \$c (@cols) {
\$str .= ";," . (join "," => map {"\$c\$_"} @rows) . ",\n";
\$re  .= ";.*,(\\w+),.*\n";

next if \$c eq 'a';

map {\$str .= "\$_:," . join (",," => free (\$_)) . ",\n"} map {"\$c\$_
+"} @rows;

my \$C = a2i \$c;

\$re .= "[^;]*\\\$C:" . join ("" => map {".*,\\\$_,"} 1 .. \$C - 1) .
+"[^;]*\n";
}

if (\$print || \$Print) {
print "'\$str' =~ \n/^\$re/\n";
exit if \$Print;
}

if (my @a = \$str =~ /^\$re/) {
print "[@a]\n";
}

__END__