I won't assume that "b is a variable" doesn't imply that "c" isn't a regex pattern instead of just a fixed string. For my example, I'll use your use of the term "body" to make a wild guess.

my $a= qr{<table[^>]*>};
my $c= qr{<tr[^>]*>};
my $d= qr{</table>};
for(  $body  ) {
    die "No $a"
        if  ! /$a/g;
    my $start= pos();
    die "No 1st $c"  if  ! /$c/g;
    die "No 2nd $c"  if  ! /$c/g;
    die "No 3rd $c"  if  ! /(?=$c)/g;
    my $end= pos();
    pos()= $start;
    die "No 3rd $c before $d"
        if  /$d/g  &&  pos() < $end;
    return substr( $_, $start, $end-$start );
}
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Also, the first two responses don't enforce "first instance of $a" and so may cause problems if the requirements have the temerity to evolve (oh, I also pre-evolved them for you in my example). ;)

P.S. My real (wild) guess is that you are parsing e-mail bodies.