$ perl -Mstrict -Mwarnings -le 'my $x = "-"; print $x =~ m-[ /\\-]- ? 1 : 0'
Unmatched [ in regex; marked by <-- HERE in m/[ <-- HERE  /\\/ at -e line 1.

$ perl -Mstrict -Mwarnings -le 'my $x = "-"; print $x =~ m-[ /\\\-]- ? 1 : 0'
1

$ perl -Mstrict -Mwarnings -le 'my $x = "-"; print $x =~ m![ /\\-]! ? 1 : 0'
1