Linux's https://man7.org/linux/man-pages/man2/execve.2.html says:

All process attributes are preserved during an execve(), except the following:

• The dispositions of any signals that are being caught are reset to the default (signal(7)).

• [...]

So a caught signal such as $SIG{ PIPE } = \&handler; gets reset to $SIG{ PIPE } = "DEFAULT";, but not $SIG{ PIPE } = "IGNORE";.

Most if not all reset things are out of necessity. $SIG{ PIPE } = \&handler can't be kept since &handler will stop existing.