First of all, it's a bad idea to use the variable names $a and $b in your code because they're special variables used by sort.

Secondly, you can write this:

foreach (@getcert) {
  $c = $_;
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...as this:

foreach my $c (@getcert) {
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To get your arrays into a hash, I'd use List::MoreUtils.

use List::MoreUtils qw( mesh );
my @users = qw( alice bob charlie );
my @ids   = qw( 123 456 789 );
my %id_of = mesh @users, @ids;
# now $id_of{ 'alice' } == 123
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If you don't want to install List::MoreUtils, you could just paste in the source for mesh:

sub mesh (\@\@;\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\@\
+@\@\@) {
    my $max = -1;
    $max < $#$_  &&  ($max = $#$_)  for @_;

    map { my $ix = $_; map $_->[$ix], @_; } 0..$max; 
}
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Lastly, if you haven't already, consider "use strict;use warnings;".