Common mistake 1

 %(var)

instead of

 %(var)s

Common mistake 2 (a bit more newbish)

 sprintfn $format, %hash, ...;

instead of

 sprintfn $format, \%hash, ...;

Alternative hashes

 %h = (%h1, %h2, %h3); # or use some hash merging module
 printfn $format, \%h, ...;

or create a tied hash which can consult hashes for you:

 tie %h, 'Your::Module', \%h1, \%h2, \%h3;
 printfn $format, \%h, ...;

sprintfn $fmt, \%hash, ...

If hash is given, sprintfn() will look for named parameters in argument and supply the values from the hash. Named parameters are surrounded with parentheses, i.e. "(NAME)". They can occur in format parameter index:

 %2$d        # sprintf version, take argument at index 2
 %(two)d     # $ is optional
 %(two)$d    # same

or in width:

 %-10d       # sprintf version, use (minimum) width of 10
 %-(width)d  # like sprintf, but use width from hash key 'width'
 %(var)-(width)d  # format hash key 'var' with width from hash key 'width'

or in precision:

 %6.2f       # sprintf version, use precision of 2 decimals
 %6.(prec)f  # like sprintf, but use precision from hash key 'prec'
 %(width).(prec)f
 %(var)(width).(prec)f

The existence of formats using hash keys will not affect indexes of the rest of the argument, example:

 sprintfn "<%(v1)s> <%2$d> <%d>", {v1=>10}, 0, 1, 2; # "<10> <2> <0>"

Like sprintf(), if format is unknown/erroneous, it will be printed as-is.

There is currently no way to escape ")" in named parameter, e.g.:

 %(var containing ))s

printfn $fmt, ...