grep:
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The grep operator taking list of values and a "testing expression". It takes one value after another and put it into the $_ then evaluate the testing expression in a scalar context. If expression evaluates to a true value it will return the $_ into the output list. In a list context grep return the list of selected items. In a scalar context grep return the number of selected items.

map:
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The map operator has a very similar step to grep operator and shares a lot of the same operational steps. It is also temporarily place the items from the list into $_ one at a time. The map operator evaluates the expression in a list context( not a scalar context like grep). Each evaluation of the expression gives a portion of the many results. the overall result in the list concatenation of all individual results. map return no.of elements that are returned in a list context.

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