input{ch1.tex}
input{ch2.tex}
...

$ cat file1.txt
foo
bar
qweinput{file2.txt}asdinput{file3.txt}zxc
baz
$ cat file2.txt
qux
quux
quuux
$ cat file3.txt
xyzzy
xyzzzy

$ <file1.txt  perl -pe 's/\input{([^}]*)}/open $1,"<$1";join("",<$1>)/ge'
foo
bar
qwequx
quux
quuux
asdxyzzy
xyzzzy
zxc
baz

gawk '
    match($0, /\input{([^}]+)/, a) {
        system("cat " a[1])
        next
    }
    {print}
'