sure vinoth, an alteration of single characters should be a character class :)

and we're on our way with the help from Basic debugging checklist

use Data::Dump qw/ dd /;
my $string = 'and a one and a two and a panda is a animal';
$string =~ (s/a (a|e|i|o|u)/an /g); 
dd( $string );
__END__
"and an ne and a two and a pandan s an nimal"
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