Rather than making two copies of the full file content in memory (and wasting cycles on a useless join), it would be better either to slurp the whole file into a scalar, like this:

{
  local $/ = undef;
  open(FILE,$filename) or die "open failed on $filename: $!";
  $wholetext = <FILE>;
  close FILE;
}
if ( $wholetext =~ /\b$searchword\b/ ) {
     # do something
}
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Or to read the file into an array and use grep on the array, like this:

open( FILE, $filename ) or die "yadda yadda";
@data = <FILE>;
if ( grep /\b$searchword\b/, @data ) {
    # do something;
}
[download]