If no string is specified via the =~ or !~ operator, the $_ string is transliterated.

##</code><code>##

C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[a][]"

C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[b][]"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[bb][]"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[bc][]"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[b][]c"

C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[b][]d"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[b][]"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[a][]c"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[ab][]"

C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[a][]"

C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[b][]"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[c][]"
1
C:\test>perl -wle"$_ = 'aaa'; print $_ !~ tr[bc][]"
1
C:\test>perl -wle"$_ = 'aab'; print $_ !~ tr[bc][]"

##</code><code>##

perl -wle"$_ = 'aaa'; $_ !~ tr[a][]d; print"