See perldoc -f sprintf; that's correct; the sprintf doc covers printf formatting options in some detail, including (just 'for example'),

precision, or maximum width
You can specify a precision (for numeric conversions) or a
maximum width (for string conversions) by specifying a "."
followed by a number.  For floating point formats, with the
exception of 'g' and 'G', this specifies the number of
decimal places to show (the default being 6), e.g.:

# these examples are subject to system-specific variation
    printf '<%f>', 1;    # prints "<1.000000>"
    printf '<%.1f>', 1;  # prints "<1.0>"
    printf '<%.0f>', 1;  # prints "<1>"
    printf '<%e>', 10;   # prints "<1.000000e+01>"
    printf '<%.1e>', 10; # prints "<1.0e+01>"

For 'g' and 'G', this specifies the maximum number of
digits to show, including prior to the decimal point as
well as after it...
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and

For floating point conversions ("e f g E F G"), numbers are
usually assumed to be the default floating point size on
your platform (double or long double), but you can force
'long double' with "q", "L", or "ll" if your platform
supports them
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I have heard that "%_f" does this in C.

No, that's not C.

I do want variable-length output

s/\.?0+\z// if /\./;
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 $x =~ s/^(.*[.].*[^0]|.*[.])0*$/$1/; $x =~ s/\.$//;
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