TIMTOWTDI - chomp in a map and join seems fairly concise yet readable.

$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
Line 1
Line 2
Line 3
EOD

my $wholeText = join q{ }, map { chomp; $_ } <$inFH>;

say $wholeText;'
Line 1 Line 2 Line 3
$
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I hope this is of interest.