Tested using -pe rather than -nle as the -p flag does an implicit print whereas -n does not.

knoppix@Microknoppix:~$ perl -E '$str .= chr int rand 256 for 1 .. 100
+00; print $str; substr $str, 4809, 10, q{P200000976}^C> xxxxx
knoppix@Microknoppix:~$ perl -E '
> $str .= chr int rand 256 for 1 .. 10000;
> substr $str, 4809, 10, q{P200000976};
> print $str;' > orig
knoppix@Microknoppix:~$ hexdump -C orig | egrep '12[cd]0'
000012c0  29 72 ff 9b 1d bf 35 39  3d 50 32 30 30 30 30 30  |)rÿ..¿59=
+P200000|
000012d0  39 37 36 43 4c 6d 90 9e  07 03 bb 42 32 c9 b5 ff  |976CLm...
+.»B2ɵÿ|
knoppix@Microknoppix:~$ perl -pe 's{P200000976}{Test123456}' < orig > 
+modified
knoppix@Microknoppix:~$ hexdump -C modified | egrep '12[cd]0'
000012c0  29 72 ff 9b 1d bf 35 39  3d 54 65 73 74 31 32 33  |)rÿ..¿59=
+Test123|
000012d0  34 35 36 43 4c 6d 90 9e  07 03 bb 42 32 c9 b5 ff  |456CLm...
+.»B2ɵÿ|
knoppix@Microknoppix:~$
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