push @{$subjects{$subject}}, %info;
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The elements of the array need to be hashrefs, so do this instead:

 push @{$subjects{$subject}}, \%info;
 print "email: " . $subjects{$subject}->[0]->{email};
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Please use strict; and use warnings .. the line my @ar = @{subjects{$key}}; is missing a "$" .. also note you can write that output as:

foreach my $key (sort (keys %subjects)){
  print "\n\nKEY = $key";
  print "\nSize of Array = " . scalar @{$subjects{$key}};
}
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