A solution using splits and maps to build a hash. The order of the output might be problematic if you desire something other than sorted.

johngg@shiraz ~/perl/Monks $ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
abcd    723-724
abcde    552-554-553
abcdef    756
EOD

my %hash =
   map { $_->[ 0 ] => [ split m{-}, $_->[ 1 ] ] }
   map { [ split ] }
   <$inFH>;

foreach my $key ( sort keys %hash )
{
    say qq{$key $_} for @{ $hash{ $key } };
}'
abcd 723
abcd 724
abcde 552
abcde 554
abcde 553
abcdef 756
[download]

I hope this is useful.

Update: This version dispenses with the hash so items will be output in the same order as input.

johngg@shiraz ~/perl/Monks $ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
abcd    723-724
abcde    552-554-553
abcdef    756
EOD

say qq{@$_} for
   map {
      my $key = $_->[ 0 ];
      my @nos = split m{-}, $_->[ 1 ];
      map { [ $key => shift @nos ] } 1 .. scalar @nos;
   }
   map { [ split ] }
   <$inFH>;'
abcd 723
abcd 724
abcde 552
abcde 554
abcde 553
abcdef 756
[download]

Update 2: Even simpler.

johngg@shiraz ~/perl/Monks $ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
abcd    723-724
abcde    552-554-553
abcdef    756
EOD

say qq{@$_} for
   map {
       my( $key, $rest ) = split;
       my @nos = split m{-}, $rest;
       map { [ $key => shift @nos ] } 1 .. scalar @nos;
   } <$inFH>;'
abcd 723
abcd 724
abcde 552
abcde 554
abcde 553
abcdef 756
[download]