In a character set,  "\1" is the character whose ASCII value is octal 01. It matches the same character (in a character set) as  "\x01" does. Outside a character set, it would be a backreference to the first capture group.

>perl -wMstrict -le
"print '1. no match octal 01'  if qq{\1} =~ m{ [^\1] }xms;
 print '2. match numeral 1'    if qq{1}  =~ m{ [^\1] }xms;
 print '3. match octal 01'     if qq{\1} =~ m{ [\1]  }xms;
 print '4. no match numeral 1' if qq{1}  =~ m{ [\1]  }xms;
"
2. match numeral 1
3. match octal 01
[download]