Trailing non-digits are fine - Perl will ignore them when converting strings to numbers (not just Perl, atoi() and friends allow that too). And if a newline is following the number, Perl won't even emit a warning:

#!/usr/bin/perl

use strict;
use warnings;

my @array;
my $number = "3\n"; 

push @array, sprintf "%.5f", "$number\n";

print $array [0], "\n";

__END__
3.00000
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Abigail