my %prices;

# read all files:
for $fn (@files){
   open (my $file, '<', $fn) or die "Can't open '$fn' for reading: $!";
   while (<$file>){
      chomp;
      my @items = split m/;/, $_;
      push @{$prices{$items[0]}, $items[1];
   } 
}

# print output:
for my $k (sort keys %prices){
    print "$k;", join(';', @{$prices{$k}}), $/;
}