For that matter, if the "..." is the same in your two conditions, I'd do it like this:

my $val = ( $EXPR ) ?  $x : $y;
my $var = sprintf( "...", $val );
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OTOH, if the "..." is different in each case, I might take the trouble to put that into a hash...

my %format = ( $x => "... for x", $y => "... for y" );
my $val = ( $EXPR ) ?  $x : $y;
my $var = sprintf( $format{$val}, $val );
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And I agree with you and with earlier replies: it's a matter of personal preference for those of us who feel a sense of "semantic unity" in certain conditional assignments.