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]