#! perl -slw
use strict;
use Data::Dumper;

my %data;
my $current_key;

while( <DATA> ) {
	my ($key, $value) = m[(^[\w]{2})?\s+(.*$)];
	if( defined $key ) {
		last if $key eq 'ER';
		push @{ $data{$key} }, $value;
		$current_key = $key;
	}
	else {
		push @{ $data{$current_key} }, $value;
	}
}

print Dumper \%data
__DATA__
T1 Line1
T2 Line2
   Line3
   Line4
T3 Line5
   Line6
ER