use strict;

my $val =
    'http://adserver.adtech.de/'
    . '?addyn|2.0|323|91793|1|277|target=_blank';

my $re = qr/
    .*\|.*\|.*\|.*\|.*\|
    (.*)
    (?=\|.*) # look-ahead
/x;

if ( $val =~ m/$re/g ) {
    print( "found: \"$1\"\n" );
    print( "pos is: " . scalar( pos( $val ) ) . "\n" );
    print( "behind pos is: \"" .
        substr( $val, pos( $val ) - length( $1 ), length( $1 ) )
        . "\"\n"
    );

    # perform substitution here using calculated offsets
    substr( $val, pos( $val ) - length( $1 ), length( $1 ) ) = "moo";
    print( "$val\n" );
}