# load file into an array of lines, and use "Line 3":

my @lines = <FILE>;
my ( $keeper ) = grep /^Line 3 : /, @lines;

# or just get "Line 3" from the file, and skip the rest:
#my ($keeper) = grep /^Line 3 : /, <FILE>;

# (update: added parens around $keeper, as per Aristotle's correction)

# either way, remove the unwanted content from the kept line:

$keeper =~ s/Line 3 : //;