Fun!

#! perl -slw
use strict;
use Data::Dump qw[ pp ];

sub x;
sub x{
    my $ref = shift;
    my @keys = sort keys %{ $ref };
    return unless @keys;
    return join ',', map {
        my $leafs = x( $ref->{ $_ } );
        $leafs ? "($leafs)$_" : $_
    } @keys;
}

my %tree;

while( <DATA> ) {
    chomp;
    my $key;
    my $ref = \%tree;
    my @nodes = split ';', $_;
   $key = shift @nodes, $ref = $ref->{ $key } //= {} while @nodes;
}
pp \%tree;

print x( \%tree );

__DATA__
a;
a;b;
a;b;e;
a;b;f;
a;c;
a;c;g;
a;c;h;
a;c;h;i;
a;c;h;j;
a;d;
[download]

Outputs

C:\test>815486
{
  a => {
         b => { e => {}, f => {} },
         c => { g => {}, h => { i => {}, j => {} } },
         d => {},
       },
}

((e,f)b,(g,(i,j)h)c,d)a
[download]

strategy
1. open file
2. read line by line
3. split line into values, and stuff into Tree object
4. close file
5. walk tree and output into desired format
6. ...
7. ...
8. profit :)