Remember that perl array indices start at zero. April would have index 3, not 4. Also you are missing August and September.

my @months = qw( January February March April May June July August Sep
+tember October November December );

f(3); # Apr
f(0); # Jan
f(11); # Dec

sub f {
    my $i = shift;  
    print join(',',  @months[$i-2, $i-1, $i]), "\n";
}
[download]

Output

February,March,April
November,December,January
October,November,December
[download]