As a one-liner:

perl -ple'$\ = $.%5 ? ";" : $/' infile > outfile
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In-place:

perl -i.bak -ple'$\ = $.%5 ? ";" : $/' file
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If it wasn't a one-liner, I wouldn't use $\

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