$ date -R
Fri, 29 Nov 2019 23:52:02 +0100
$ perl -MO=Deparse friday.pm > /dev/null
friday.pm syntax OK
⋮
$ date -R
Sat, 30 Nov 2019 00:03:06 +0100
$ perl -MO=Deparse friday.pm > /dev/null
syntax error at friday.pm line 9, at EOF
friday.pm had compilation errors.