Net::CIDR::Lite does this quite easily:

my $cidr = Net::CIDR::Lite->new();
while (<DATA>) {
  chomp;
  $cidr->add_range($_);
}
for $r ( $cidr->list_range() ) {
  print "$r\n";
}
__DATA__
5.0.0.0-5.0.0.255
5.0.1.0-5.0.1.255
7.15.1.0-7.15.6.255
7.15.7.0-7.15.9.255
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