$ perl -e '
    my $use_grep = 1;
    my @x = qw{a b a c bab aca};
    my @y = map { /(.?a.?)/ ? $use_grep ? $_ : $1 : () } @x;
    print "|$_|\n" for @y;
'
|a|
|a|
|bab|
|aca|