The first line can be rewritten as:

foreach my $data (@Clec_id) {
    $count{$data}++;
}
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Every time the same peice of data appears in the array, that index key is incremented. Thus, if the string 'foo' appears three times in the array, then $count{foo} will be three after the loop runs.

The second line is simply dumping the contents of the hash to STDOUT using a similar construct to the line above it.