$ perl -Mstrict -Mwarnings -E '
> open my $fi, q{<}, \ <<EOFi or die $!;
> File I Line 1
> File I Line 2
> File I line 3
> EOFi
> 
> open my $fa, q{<}, \ <<EOFa or die $!;
> File A Line 1
> File A Line 2
> EOFa
> 
> open my $fb, q{<}, \ <<EOFb or die $!;
> File B Line 1
> File B Line 2
> File B Line 3
> EOFb
> 
> open my $fc, q{<}, \ <<EOFc or die $!;
> File C Line 1
> File C Line 2
> File C Line 3
> File C Line 4
> EOFc
> 
> my @slaveFHs = ( $fa, $fb, $fc );
> while ( <$fi> )
> {
>     chomp;
>     say join q{,}, $_,
>        map { chomp; $_ }
>        map { eof $_ ? q{--eof--} : scalar <$_> }
>        @slaveFHs;
> }'
File I Line 1,File A Line 1,File B Line 1,File C Line 1
File I Line 2,File A Line 2,File B Line 2,File C Line 2
File I line 3,--eof--,File B Line 3,File C Line 3