What's happening is that even if $mesg->entries returns multiple entries, you overwrite $firstName etc. each time through the loop, so your single print is the last entry received. Instead, store the info in a handy hash and then you can print a nice sorted list (or whatever):

my ($acct,%records);
foreach $entry ($mesg->entries) {
   $firstName = $entry->get_value("firstName");
   $lastName = $entry->get_value("lastName");
   $accountName = $entry->get_value("accountName");
   $accountNumber = $entry->get_value("accountNumber");
   $records{$accountName} = [ $firstName, $lastName, $accountNumber ];
}
foreach $acct (sort keys %records) {
    print $acct,": ",join(' ',@{$records{$acct}}),"\n";
}
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HTH, SSF