my @a = ( 42, 42, 43, 43, 43, 44, 45, 46 );
my @b = ( 43, 45 );

for my $i ( 0 .. $#b ) {
    for my $j ( 0 .. $#a ) {
        next unless defined $a[ $j ];
        $a[ $j ] = undef, last if $a[ $j ] == $b[ $i ];
    }
}
@a = grep { defined } @a;
print join ', ', @a; print "\n";