$ perl -le'my @x = ( "A", "A, B", "A, B, C" );
# 1a
for my $test ( @x ) {
    $test =~ s/(.*),/$1 and/s;
    print $test;
    }
'
A
A and B
A, B and C

$ perl -le'my @x = ( "A", "A, B", "A, B, C" );
# 1b
for my $test ( @x ) {
    $test =~ s/.*\K,/ and/s;
    print $test;
    }
'
A
A and B
A, B and C

$ perl -le'my @x = ( "A", "A, B", "A, B, C" );
# 2a
for my $test ( @x ) {
    $test =~ s/,([^,]*)\z/ and$1/;
    print $test;
    }
'
A
A and B
A, B and C

$ perl -le'my @x = ( "A", "A, B", "A, B, C" );
# 2b
for my $test ( @x ) {
    $test =~ s/,(?=[^,]*\z)/ and/;
    print $test;
    }                     
'
A
A and B
A, B and C

$ perl -le'my @x = ( "A", "A, B", "A, B, C" );
# 3
for my $test ( @x ) {
    $test = reverse $test;
    $test =~ s/,/dna /;
    $test = reverse $test;
    print $test;
    }
'
A
A and B
A, B and C