From type/Allomorph

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method trim-trailing(Allomorph:D:)

Calls Str.trim-trailing on the invocant's Str value.

From type/Cool

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sub trim-trailing(Str(Cool))
method trim-trailing()

Coerces the invocant (or in sub form, its argument) to Str, and returns the string with trailing whitespace stripped.

my $stripped = '  abc '.trim-trailing;
say "<$stripped>";          # OUTPUT: «<  abc>␤»

From type/Str

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method trim-trailing(Str:D: --> Str)

Removes the whitespace characters from the end of a string. See also trim.