Consider this approach:

use warnings;
use strict;

my $prev;
while(<DATA>) {
    if (/;/) {
        chomp;
        $prev = $_;
    }
    else {
        s/(\d{5})/$1;/;
        print "$prev; $_";
    }
}

__DATA__
Some Company; Some Street 52
12345 City1
Another Company; Another Street 63
23456 City2
Yet Another Company; New Street 11
34546 City3
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