You were sorting a three element array containing a name, a city, and an age. The fact that you were trying to sort the array before completely reading in the file should have been a dead giveaway. You need to create an Array of Arrays (AoA), as described in perllol.

chomp removes the newline.

#!/usr/local/bin/perl
use strict;
use warnings;

my $file = "input.log";
open(IN, $file) || die "Couldn't find '$file': $!";

my @age;
while (my $line = <IN>) {
    chomp($line);
    my ($name, $age) = split(/\s*,\s*/, $line);
    push(@age, [ $name, $age ]);
}
close(IN);

my @sorted_ages = sort { $b->[1] <=> $a->[1] } @age;

foreach my $a (@sorted_ages){
    print $a->[0], ' is ', $a->[1], ' years old.', $/;
}
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btw, it's better to store birthdate/birthyear than age and calculate the age when needed, so that you don't have to update your records every year.