my $line = do { local $/; <$fh> };
$line =~ s/\n*\n/ /g;
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Or:

my $line = do { local $/; <$fh> };
$line =~ tr/\n/ /s;
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Or just:

( my $line = do { local $/; <$fh> } ) =~ tr/\n/ /s;
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