#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $path_to_file = 'test.txt';

open my $fh, '<', $path_to_file
    or die "Could not open " . $path_to_file . " $!\n";

chomp(my @lines = <$fh>);

close $fh
    or die "Could not close " . $path_to_file . " $!\n";

# Remove empty lines if this is desired?
@lines = grep /\S/, @lines;

my %HoA;
foreach my $line (@lines) {
    my ($match, $remaining) = split(/\|\|\|/, $line);
    push (@{$HoA{$match}}, $remaining);
}

print Dumper \%HoA;

my @updated_lines;
foreach my $key (keys %HoA) {
    my $concat_str;
    foreach my $i ( 0 .. $#{ $HoA{$key} } ) {
	if ($i == 0){
	    $concat_str .= $HoA{$key}[$i] . "; ";
	}
	$concat_str .= $HoA{$key}[$i] . " ";
    }
    # Trim white space on right
    $concat_str =~ s/\s+$//;
    push @updated_lines, $key.$concat_str.'|||';
}

print Dumper \@updated_lines;

__END__

$ perl test.pl
$VAR1 = {
          '21997' => [
                       '70049,,20170428154818,20170527235959',
                       '70070,,20170428154739,20170527235959'
                     ],
          '21998' => [
                       '70049,,20170428154818,20170527235959',
                       '70070,,20170428154739,20170527235959',
                       '70071,,20170428154739,20170527235959'
                     ]
        };
$VAR1 = [
          '2199770049,,20170428154818,20170527235959; 70049,,20170428154818,20170527235959 70070,,20170428154739,20170527235959|||',
          '2199870049,,20170428154818,20170527235959; 70049,,20170428154818,20170527235959 70070,,20170428154739,20170527235959 70071,,20170428154739,20170527235959|||'
        ];