my @phrases = qw( foo bar );
my $sentence = " foo bar ";

my ($phrases_pat) = map qr/$_/, join '|', map quotemeta, @phrases;

$sentence =~ s/($phrases_pat)(\s)($phrases_pat)/$1$2$2$3/g;
$sentence =~ s/(\s$phrases_pat\s)/#$1#/g;

print("$sentence\n");   # "# foo ## bar #"