use strict;
use warnings;
use Data::Dumper;

my %predec;

while (<DATA>) {
	chomp;
	my ($succ, $pred) = split /;/, $_;
	$pred = $succ if $pred eq '';
	push @{$predec{$pred}}, $succ;
}
my $continue = 1;
while ($continue) {
	$continue = 0;
	for my $pred (keys %predec) {
		my $succ = @{$predec{$pred}}[-1];	
		if (defined $succ and exists $predec{$succ} and scalar @{$predec{$succ}} != 0) {
			next if $succ eq $pred;
			push @{$predec{$pred}}, @{$predec{$succ}};
			delete $predec{$succ};
			$continue = 1;
		}
		$succ = @{$predec{$pred}}[0];
		if (defined $succ and exists $predec{$succ} and scalar @{$predec{$succ}} != 0) {
			unshift @{$predec{$pred}}, @{$predec{$succ}};
			delete $predec{$succ};
			$continue = 1;
		}
	}
}

print "Data structure: \n", Dumper \%predec;
print "\n\nResult:\n\n";

for my $key (keys %predec) {
	next if scalar @{$predec{$key}} == 0;
	print "$key => $key\n" and next if $key eq @{$predec{$key}}[0];
	print "$key => $key\n";
	print "$_ => $key\n" for @{$predec{$key}};
}

__DATA__
567;456
456;345
345;234
234;123
339;228
228;117
131;
435;324
324;213
372;


##</code><code>##


$ perl  predecessor.pl
Data structure:
$VAR1 = {
          '123' => [
                     '234',
                     '345',
                     '456',
                     '567'
                   ],
          '131' => [
                     '131'
                   ],
          '234' => [],
          '213' => [
                     '324',
                     '435'
                   ],
          '345' => [],
          '324' => [],
          '372' => [
                     '372'
                   ],
          '117' => [
                     '228',
                     '339'
                   ]
        };


Result:

123 => 123
234 => 123
345 => 123
456 => 123
567 => 123
131 => 131
213 => 213
324 => 213
435 => 213
372 => 372
117 => 117
228 => 117
339 => 117