I think a good approach is to break the problem down into sections and make smaller compiled regular expression building blocks then put them together for the final solution. Hopefully, I've understood the requirement but the following seems to do the trick.

#!/usr/local/bin/perl -w
#
use strict;

# Make some building blocks. What we want at the start; what we don't 
+want
# after "foobar:" using a negative look-ahead assertion; any five
# characters; what we want at the end (which we will discard later).
#
our $rxFooBar = qr{foobar:};
our $rxNotGonkish = qr{(?!(?:g(?:o(?:n(?:k)*)*)*|@))};
our $rxFive = qr{.{1,5}};
our $rxGonk = qr{(?:gonk|@)};

# Put the building blocks together using "()" to capture everything ex
+cept
# the "gonk" or "@" at the end;
#
our $rxPutItAllTogether = qr{($rxFooBar$rxNotGonkish$rxFive)$rxGonk};

while(<DATA>)
{
    chomp;
    print "$_\n";
    print /$rxPutItAllTogether/ ? "$1\n\n" : "Failed\n\n";
}

__END__
foobar:hellogonk
foobar:gonk
foobar:higonk
foobar:helloworldgonk
foobar:ggonk
foobar:gogonk
foobar:gongonk
foobar:gonkgonk
foobar:gonkygonk
foobar:@gonk
foobar:its@
foobar:toomany@
[download]

Which produces the following output.

foobar:hellogonk
foobar:hello

foobar:gonk
Failed

foobar:higonk
foobar:hi

foobar:helloworldgonk
Failed

foobar:ggonk
Failed

foobar:gogonk
Failed

foobar:gongonk
Failed

foobar:gonkgonk
Failed

foobar:gonkygonk
Failed

foobar:@gonk
Failed

foobar:its@
foobar:its

foobar:toomany@
Failed
[download]

Cheers,

JohnGG