This is an assignment (using =) clobbering $_ with an empty array. An assignment of an undefined ... Similar to if ($_ = ()) or if ($_ = undef).

$_ is clobbered with the true/false result of evaluating the match of the regex against $_:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -e
"print 'Enter a string and I will determine the data type: ';
 chomp($_ = <STDIN>);
 dd 'A:', $_;
 ;;
 if ($_ = (m/^[\$]\w/)) {
    print qq{\n'$_' is a scalar data type\n};
    }
 dd 'B:', $_;
 ;;
 if ($_ = (m/^[\@]\w/)) {
    print qq{\n'$_' is an array data type\n};
    }
 dd 'C:', $_;
"
Enter a string and I will determine the data type: $scalar
("A:", "\$scalar")

'1' is a scalar data type
("B:", 1)
("C:", "")

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -e
"print 'Enter a string and I will determine the data type: ';
 chomp($_ = <STDIN>);
 dd 'A:', $_;
 ;;
 if ($_ = (m/^[\$]\w/)) {
    print qq{\n'$_' is a scalar data type\n};
    }
 dd 'B:', $_;
 ;;
 if ($_ = (m/^[\@]\w/)) {
    print qq{\n'$_' is an array data type\n};
    }
 dd 'C:', $_;
"
Enter a string and I will determine the data type: @ra
("A:", "\@ra")
("B:", "")
("C:", "")
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