I solved this by removing the global identifier. !!!!!!!!!!!!

No, you solved it by making the incomplete statement
    $function_call = "memcpy(a, b, sizeof(a));"
(no ; terminates the statement) into a complete statement (by adding a ; and then not mentioning it to anyone, possibly even including yourself ;).

c:\@Work\Perl>perl -wMstrict -le
"my $function_used = 'memcpy';
 my $function_call = 'memcpy(a, b, sizeof(a));';
 $function_call =~ s/$function_used//g;
 print qq{'$function_call'};
"
'(a, b, sizeof(a));'
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(I'm assuming that by 'problem', you mean the fact that the OPed code will not compile.)

Update: If there's any chance that  $function_used may contain regex metacharacters, it's a very good idea to use quotemeta on it at some point, e.g.:
    $function_call =~ s/\Q$function_used\E//g;

Nope, I doubt this! Even without being able to test the code you posted. (did you?)

memcpy should be deleted now.

My best guess is that in your real code $function_used doesn't contain what you think it should.

Maybe a problem with scoping?

> So how can one solve this? !!!!!!!!!!!! I solved this by removing the global identifier. !!!!!!!!!!!!

Your keyboard is broken, maybe that's the problem?(...??????????)

:)

update

now I can test, the code you posted explicitly informs you about the wrong semicolon:

> perl
$function_used = "memcpy";
$function_call = "memcpy(a, b, sizeof(a));"
$function_call =~ s/$function_used//g;
Scalar found where operator expected at - line 3, near "$function_call
+"
    (Missing semicolon on previous line?)
syntax error at - line 3, near "$function_call "
Execution of - aborted due to compilation errors.
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and the corrected code doesn't have any of your imagined "problems"

> perl
$function_used = "memcpy";
$function_call = "memcpy(a, b, sizeof(a))";
$function_call =~ s/$function_used//g;
print $function_call;
__END__
(a, b, sizeof(a))
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so I think you should better take a look into How (Not) To Ask A Question :)

perl -e 'sub foo {return qq|abracadabra|}; my $line = qq|hi there: sub
+|; if($line=~s/sub/&foo/eg){print $line};'
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Prints

hi there: abracadabra
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