Yes, There's More Than One Way To Do It but there must be a canonical or most efficient or most concise way. I'll add answers I know of and see what percolates to the top.
To be clear, the question is how best to read the contents of a file into a string.
One solution per answer.
How about this:
use File::Slurp; my $text = read_file($filename);
ETA: note Bug #83126 for File-Slurp: Security hole with encoding(UTF-8). I now recommend using File::Slurper (disclaimer: I wrote it), also because it has better defaults around encodings:
use File::Slurper; my $text = read_text($filename);
use Path::Tiny; path($filename)->slurp_utf8;