You can't set the input delimeter unless you know more than you've told us about comment lines.

I'd do something like:

my $full_line = '';
while ( my $line = <FH> )
{
    chomp $line;
    next if substr( $line, 0, 2 ) eq '//';
    $full_line .= " $line"; # space wanted? your call!
    next unless substr( $line, -1, 1 ) eq ';';

    #do something with $full_line
    print "$full_line\n";

    $full_line = '';
}
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Update: replace the $full_line .= line with

    $full_line .= $full_line eq '' ? $line : " $line";
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Update^2: I should have said this has been tested and produced the following with your sample data:

some code;
more code;
some very long long long code;
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