Try:

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;

for(split /\n/, << 'END')
jay 21 34 56
bob 12 39
jay 11 10
bob 14
jay 190
bob 13
END
 {s/jay/bob/ or s/bob/jay/;
  say
 }
[download]

Produces

bob 21 34 56
jay 12 39
bob 11 10
jay 14
bob 190
jay 13

Another method that reads the data in paragraph mode, splits into first element and remaining elements within lines in one map then swaps the first elements of the two lines and rejoins things up in a second, finally joining the pairs of lines into paragraphs again.

$ perl -Mstrict -Mwarnings -E '
> open my $fh, q{<}, \ <<EOF or die $!;
> jay 21 34 56
> bob 12 39
> 
> jay 11 10
> bob 14
> 
> jay 190
> bob 13
> EOF
> 
> say join qq{\n\n}, do {
>    local $/ = q{};
>    map {
>        ( $_->[ 0 ]->[ 0 ], $_->[ 1 ]->[ 0 ] )
>        =
>        ( $_->[ 1 ]->[ 0 ], $_->[ 0 ]->[ 0 ] );
>        join qq{\n}, map { join q{ }, @$_ } @$_;
>    }
>    map {
>        chomp;
>        [ map { [ split m{\s}, $_, 2 ] } split m{\n} ];
>    }
>    <$fh>;
>    };'
bob 21 34 56
jay 12 39

bob 11 10
jay 14

bob 190
jay 13
$
[download]

I hope this is of interest.

 cut -f2- -d ' ' list.txt| sed '1~2 s/^/bob /' | sed '2~2 s/^/jay /'
[download]