Here's the script tweaked to save, delete and restore the reg key:

echo on

reg export HKLM\Software\Classes\.pl saved.reg /y
reg delete HKLM\Software\Classes\.pl /f

assoc .pl=perltest
perl -e"system(qq(ftype perltest=$^X \"%%1\" %%*))"
ftype perltest

mkdir pipe
cd pipe
pipe

perl -e "print 'print qq(| $_) while <>'" > pipe.pl
set pathext=.pl;.com;.exe;.bat;.cmd;
pipe pipe.pl
pipe < pipe.pl
type pipe.pl | pipe

set pathext=.com;.exe;.bat;.cmd;
pipe pipe.pl
pipe < pipe.pl

set pathext=.com;.exe;.bat;.cmd;.pl;
pipe pipe.pl
pipe < pipe.pl
type pipe.pl | pipe

del pipe.pl
cd ..
rmdir pipe

reg delete HKLM\Software\Classes\.pl /f
reg import saved.reg
del saved.reg
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And here the results of a run on my system (no difference that I can see):

c:\test>pipetest

c:\test>echo on

c:\test>reg export HKLM\Software\Classes\.pl saved.reg /y
The operation completed successfully.

c:\test>reg delete HKLM\Software\Classes\.pl /f
The operation completed successfully.

c:\test>assoc .pl=perltest
.pl=perltest

c:\test>perl -e"system(qq(ftype perltest=$^X \"%1\" %*))"
perltest=C:\Perl64\bin\perl.exe "%1" %*

c:\test>ftype perltest
perltest=C:\Perl64\bin\perl.exe "%1" %*

c:\test>mkdir pipe

c:\test>cd pipe

c:\test\pipe>pipe
'pipe' is not recognized as an internal or external command,
operable program or batch file.

c:\test\pipe>perl -e "print 'print qq(| $_) while <>'"  1>pipe.pl

c:\test\pipe>set pathext=.pl;.com;.exe;.bat;.cmd;

c:\test\pipe>pipe pipe.pl
| print qq(| $_) while <>
c:\test\pipe>pipe  0<pipe.pl
| print qq(| $_) while <>
c:\test\pipe>type pipe.pl   | pipe
| print qq(| $_) while <>
c:\test\pipe>set pathext=.com;.exe;.bat;.cmd;

c:\test\pipe>pipe pipe.pl
'pipe' is not recognized as an internal or external command,
operable program or batch file.

c:\test\pipe>pipe  0<pipe.pl
'pipe' is not recognized as an internal or external command,
operable program or batch file.

c:\test\pipe>set pathext=.com;.exe;.bat;.cmd;.pl;

c:\test\pipe>pipe pipe.pl
| print qq(| $_) while <>
c:\test\pipe>pipe  0<pipe.pl
| print qq(| $_) while <>
c:\test\pipe>type pipe.pl   | pipe
| print qq(| $_) while <>
c:\test\pipe>del pipe.pl

c:\test\pipe>cd ..

c:\test>rmdir pipe

c:\test>reg delete HKLM\Software\Classes\.pl /f
The operation completed successfully.

c:\test>reg import saved.reg
The operation completed successfully.

c:\test>del saved.reg

c:\test>
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