To quote from perlre:

The bracketing construct ( ... ) creates capture buffers. To refer to the digit'th buffer use \<digit> within the match. Outside the match use "$" instead of "\". (The \<digit> notation works in certain circumstances outside the match. See the warning below about \1 vs $1 for details.) Referring back to another part of the match is called a backreference

In other words, use

print 'dup' if /(.).*\1/;
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