Sounds like an identical question to Concatenate every 5 lines of input with a comma.. What follows is my earlier answer:

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As a one-liner:

perl -ple'$\ = $. % 5 ? ";" : $/' infile > outfile
[download]

In-place:

perl -i.bak -ple'$\ = $. % 5 ? ";" : $/' file
[download]

If it wasn't a one-liner,

while (<$fh>) {
    chomp;
    print $_, $. % 5 ? ";" : $/;
}
[download]

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