If you want to get the frequencies of all the letters in the name rather than just one in particular then use a hash and split. Here I have assumed that you are only interested in the name part of the address.

use strict;
use warnings;

my $addr = q{xqwertyxyx@gmail.com};

my ( $name ) = $addr =~ m{^([^@]+)};
my %letterFreq;
$letterFreq{ $_ } ++ for split m{}, $name;

print
   map  { qq{$_: $letterFreq{ $_ }\n} }
   sort { $letterFreq{ $b } <=> $letterFreq{ $a } || $a cmp $b }
   keys %letterFreq;
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The output.

x: 3
y: 2
e: 1
q: 1
r: 1
t: 1
w: 1
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I hope this is useful.

Cheers,

JohnGG