You could use something like

my @Book = ();
my @MatchFound = ();
my %seen;

foreach (@Book) {
  if ( /\bPerl\b/i && /\bMonks\b/i && /\bRule\b/i ) {
    push @MatchFound, $_
      unless $seen{$_}++;
  }
}
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Most often, when you're after a unique list in Perl, you want to use a hash