Why does this have to be a "one-liner"?.
Why not code something simple? Is this some kind of "golf" test?

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


while (<DATA>)
{
    process_record ($1) if /^\s*START\s*(\w+)/;  
}

sub process_record
{
   my $start_value = shift;
   my @record;
   
   my $line;
   while (defined ($line=<DATA>) and $line !~ /^\s*END/)
   {
      push @record, $line;
   }
   my ($end_value) = $line =~ /^\s*END\s+(\w+)/;
   
   @record = map{"$start_value $end_value $_"}@record;
   print @record;
   print "\n";
}

=prints
abc xyz MAY 
abc xyz JUNE 
abc xyz JULY

oue 2345 MAY 
oue 2345 JUNE 
oue 2345 JULY
=cut


__DATA__
START abc
MAY 
JUNE 
JULY
END xyz
blah
....more blah...
START oue
MAY 
JUNE 
JULY
END 2345
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Update: I looked back upon this thread and these 2 lines could have been better written. I don't remember why I did what I did in the original coding. I do remember that I wrote the code in an attempt to get the OP "unstuck" and was surprised at the "one liner" requirement. Anyway the OP obviously doesn't care about this, but other future readers might...However, I will add that in this sort of situation, the difference probably doesn't matter at all...

  # @record = map{"$start_value $end_value $_"}@record;
  # print @record;
  print "$start_value $end_value $_" for @record;
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