Did you read Symbolic-Unary-Operators?

$ perl -MO=Deparse -e 'print -"asdf"'
print '-asdf';
-e syntax OK
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If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned.

$ perl -MO=Deparse -e 'print -"1asdf"'
print -1;
-e syntax OK
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If, however, the string begins with a non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert the string to a numeric and the arithmetic negation is performed.

$ perl -MO=Deparse -e 'print -"-1asdf"'
print '+1asdf';

$ perl -MO=Deparse -e 'print -"+1asdf"'
print '-1asdf';
-e syntax OK
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Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned.