For the first part of your assignment:

use strict;

my $lastline;

while (<DATA>) {
    $lastline = $_ if /;/;
}
print $lastline;

__DATA__
This is not the right line
This is OK, it contains a ;
This is no good either
Hey, a ; in this line!
Drop this.
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Of course you will want to change the <DATA> filehandle for some proper filehandle pointing to the text file.