You can also grab all matches at once to an array:

c:\@Work\Perl>perl -wMstrict -le
"my $s = qq{xxx \@B grab \n this \@E yy zzzz \@B and \n this \n too \@
+E qq};
 print qq{[[$s]]};
 ;;
 my @captures = $s =~ m{ \@B .*? \@E }xmsg;
 print qq{<<$_>>} for @captures;
"
[[xxx @B grab
 this @E yy zzzz @B and
 this
 too @E qq]]
<<@B grab
 this @E>>
<<@B and
 this
 too @E>>
[download]

Update 1:

$string=" ... @begin ... @end ..." .....

Note that arrays like  @begin @end interpolate into interpolating or "double-quote" strings like  " ... " and  qq/ ... / along with  $scalar scalars. Neither interpolate into non-interpolating 'single-quote' strings. Please see Quote and Quote-like Operators and Quote-Like Operators in perlop.

Update 2: Or if you mean you want to grab only the sub-string between the delimiting patterns, add a coupla parens:

c:\@Work\Perl>perl -wMstrict -le
"my $s = qq{xxx \@B grab \n this \@E yy zzzz};
 print qq{[[$s]]};
 ;;
 my @captures = $s =~ m{ \@B (.*?) \@E }xmsg;
 print qq{<<$_>>} for @captures;
"
[[xxx @B grab
 this @E yy zzzz]]
<< grab
 this >>
[download]

(You could also do this without a capture group using look-arounds, but let's let this stand for now.)