you could use concatenation ("the dot operator") :

my $crypticString="LR20SR37PD30LR23CR23BT01H";
my $acceptableLength=40;
my $paddedString =$crypticString . " " x 
    ($acceptableLength - length ($crypticString));
print "'$paddedString'";
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Also, using \s to indicate " " might be confused with the character class \s (which also includes newlines and tabs).