use warnings;
use strict;

while (<>) {
    chomp;
    my ($name,@rest)= split(/\s*;\s*/,$_);
    foreach (@rest) {
    print $name,'; ',$_,"\n";
    }
}
[download]

this will print the result to the screen, but if you call this script with

thisscript old.txt > new.txt
[download]

the output gets stored in the file new.txt. Or you can change the script to explicitly open a file with the open() function and print to it