#!perl -i.bak
my $newnum = shift;
@ARGV = qw( one.txt );
undef $/; # sluurp
$_ = <>;
s/(Authentication number) \d+/$1 $newnum/;
print
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The critical piece in all that is the -i.bak option passed to perl. If you can't do it on the shebang line, do it the exec (cmd) line of perl.