You have slurped all your input lines into the 1st element of the @array variable. Here's how I would do it:

use warnings;
use strict;
use File::Slurp;

my $file  = '/etc/passwd';
my @lines = read_file($file);
chomp @lines;
my @sorted = map {$_->[0]}
             sort {$a->[1] <=> $b->[1] || $a->[1] cmp $b->[1]}
             map {[$_,(split':')[0]]} @lines;

print "$_\n" for @sorted;
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• File::Slurp
• use strict and warnings
• I changed the name of the array variable to something slightly more meaningful.