Careful, you’re invoking grep in scalar context. $keeper will only contain the count of matches. This has to be written with a parenthesised my, like so:

my ( $keeper ) = grep /^Line 3 : /, @lines;
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However, that always goes through the entire data, regardless of where the match is found. A better way would be List::Util’s first; with which the context does not matter either:

use List::Util qw( first );

my $keeper = first { /^Line 3 : / } @lines;
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Makeshifts last the longest.