# First time
next unless defined %{$matches{$fasta_id}};         # False
last unless defined @{$matches{$fasta_id}{$site}};  # False

# Second time
next unless defined %{$matches{$fasta_id}};         # Now true!!
last unless defined @{$matches{$fasta_id}{$site}};

##</code><code>##

# First time
next unless defined %{$matches{$fasta_id}};         # False -> next
last unless defined @{$matches{$fasta_id}{$site}};  # Skipped by next

# Second time
next unless defined %{$matches{$fasta_id}};         # Still false
last unless defined @{$matches{$fasta_id}{$site}};