my $string = "This is a test string containing fifty-four characters";

# for example, we'll set limit at 25
my $limit = 25;

my @lines = ();

push @lines, $_ for ($string =~  /\s*(.{0,$limit}\b)gs);

# regex explanation
\s*        - match any leading spaces
(          - start match
.          - match any character
{0,$limit} - up to $limit times
)          - end match
\b         - followed by a word boundary

# modifiers
g - global (match as many times as you can)
s - treat whole string as a single line.