Hello Shivam05, and welcome to the Monastery!

Your code has two problems. First, you need to disable the metacharacters in $grepper when using it in a regular expression:

my @LABEL = grep(/\Q$grepper\E/, @array);
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See quotemeta.

Second, as others have noted, you set the value of $array[0] to label(, which does not contain the double-quote character. Therefore, the regular expression finds nothing to match in @array. You can fix this in several ways:

$array[0] =   "label(\"";
$array[0] =   'label("';
$array[0] = qq{label("};
$array[0] =  q{label("};
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See perlop#Quote-and-Quote-like-Operators

Hope that helps,