grep can only return anything if it has something to chose from. @value starts out empty so grep can never return anything except an empty list so the if expression can never be true so @value never gets any content.

The usual way to check if something has been seen before is to use a hash:

#!/usr/bin/perl -w
use strict;

my @inputs = qw(first second third fourth fifth first);
my %found;
my @values;

for my $element (@inputs) {
    if (!$found{$element}) {
        push @values, $element;
        ++$found{$element};
    }
}

print "@values\n";
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Prints:

first second third fourth fifth
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