There are a number of techniques to deal with this kind of parsing. I know how to implement several of them and I'm ok with them all.

Your example data format is unusual because it has more than one significant complicating factor.

Just for fun, I show an alternate coding that demo's some other techniques. I make no claim about "better". There is seldom a coding pattern that works "the best" in all situations. I used your regex'es as they looked fine to me. At the end of the day, all of the "states" have to be described and handled.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
$|=1;

# Don't read in another line if we are still working
# on a START line. This is caused by the
# X START Y syntax in conjunction with the idea
# of END absent a START in this example file format.

# As a thought redefining the input separtator to
# be 'START' could possibly be productive if the format
# is not exactly like this?,

# This format has some of the nastiest things to deal
# with. They normally do not occur all at once!


my @record=();
my $line_in ='';

while ( $line_in =~ /START/ or $line_in =<DATA>)
{      
    $line_in =  construct_record($line_in) if $line_in =~ /START/;
}


sub construct_record
{
    my $line = shift;
    
    if ( (my $x) = $line =~ /START\s+(\w+)\s*$/)
    {
       push @record, $x;
    }

    while (defined ($line = <DATA>) and ($line !~ /(START|END)/) )
    {
        $line =~ s/^\s*|\s*$//g;
        push @record, $line;
    }
    
    $line //= '';  #could be an EOF
  
    if (my ($b4end) = $line =~ /^ (?: (.+) \s+ )? END $/x)
    {
        push @record, $b4end if $b4end;
        output_record();
        return '';  # no continuation of this record
    } 
    if ( my ($x,$y) = $line =~ /^ (?: (.+) \s+ )? START (?: \s+ (.+) )
+? $/x )
    {
        if ($x)
        {
           push @record, $x; 
           output_record();  
        }
        if ($y)
        {
            output_record();  # might be: "^START 77"?
            return "START $y";
        }
     }   
     return '';
}

sub output_record  # or process it somehow...
{
    print "Record: @record\n" if (@record >1);
    @record=();
}

=Prints
Record: a b
Record: c d e f g
Record: h i
Record: j k
=cut


__DATA__
START
a
b
START c
d
e
f
g END
ignoreme
START h
i START j
k
END
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