The regex expression .*\|{5} matches anything followed by exactly five '|' characters in a row, which is not what is present in the example of the OP.

A regex something like the following might be better (although it is still a bit awkward):

>perl -wMstrict -le
"my $u =
  'http://foo.bar.de/?baz|2.0|323|91793|1|277|fee=_fie';
 print $u;
 $u =~
  s{ \A ((?: [^|]* \|){5}) \d+ (\|.*) \z }{${1}976$2}xms;
 print $u;
"
http://foo.bar.de/?baz|2.0|323|91793|1|277|fee=_fie
http://foo.bar.de/?baz|2.0|323|91793|1|976|fee=_fie
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